For RDS, up to 40 Reserved instances can be reserved. Beyond that, you are required to fill the form.

Official documentation

Answer from Ruhan Sharief on Stack Overflow
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AWS
aws.amazon.com › database › amazon rds › reserved instances
Amazon RDS Reserved Instances | Cloud Relational Database | Amazon Web Services
5 days ago - Reserved Instances are available in all the AWS regions. Reserved Instances are available for all supported DB Engines. Pricing for Reserved Instances is available on the Amazon RDS Pricing page.
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Concurrencylabs
concurrencylabs.com › blog › guide-to-rds-reserved
Part II: RDS - The Ultimate Guide to Saving Money with AWS Reserved "Anything" - Concurrency Labs
Also, since RDS doesn’t offer flexibility between instance families, you have to be sure that you’ve chosen the right instance type based on resource demands from your application (i.e. disk I/O, CPU, memory, networking). That’s why it’s very important to execute load tests and monitor your application in Production before making an RDS Reserved purchase decision. ... Just like EC2 (and all AWS services that involve compute capacity), over time you can expect AWS to release new RDS DB instance classes.
Discussions

RDS Reserved Instances
According to FAQ (RDS) there is no need to chose a specific AZ when purchasing reserved instances for RDS. Is this true? https://aws.amazon.com/rds/faqs/ "Q: Will there always be reservati... More on repost.aws
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July 17, 2016
amazon web services - Reserved instance limit for EC2 and RDS - Stack Overflow
We know that we have below limit for reserved instances for EC2 There is a limit to the number of Reserved Instances that you can purchase per month. For each Region you can purchase 20 regional Re... More on stackoverflow.com
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RDS Reserved Instances
Just be aware that they are tied to instance family. More on reddit.com
🌐 r/aws
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November 25, 2023
Changing class of a reserved instance in RDS from m3 to m5?
i'm wondering if its too late to change? Yes, but also maybe not -- I would talk to your account rep or file a support ticket ASAP. They will likely be willing to fix it for you as a one-off kindness, but there are no guarantees. if i create a reserved instance with multi-az, is it possible to use it for an instance that does not use multi-az? Yup, it's actually quite flexible. Read the section titled "Reserved Instance Size Flexibility" in the RDS RI page . More on reddit.com
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January 26, 2022
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YouTube
youtube.com › watch
RDS Follow Along - Reserved Instances
Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube.
Published   November 19, 2019
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RTFM
rtfm.co.ua › home › virtualization › amazon web services › aws: cost optimization – purchasing rds reserved instances
AWS: cost optimization – purchasing RDS Reserved Instances
November 2, 2021 - The first one will be about AWS RDS Reserved Instances. The idea is quite simple: you are committing to use some types of instances for one or three years. Here, you’ll be able to choose to pay the commitment immediately for the whole term, ...
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Parquantix
parquantix.com › how to reduce amazon rds costs with reserved instances
How to Reduce Amazon RDS Costs with Reserved Instances | Parquantix
May 4, 2021 - Amazon RDS Reserved Instances are available for all DB Engines supported by AWS, including Amazon Aurora, PostgreSQL, MySQL, MariaDB, Oracle, and Microsoft SQL Server.
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AWS re:Post
repost.aws › questions › QUeYRda5a0RNWCnh5PWNt5cA › rds-reserved-instances
RDS Reserved Instances | AWS re:Post
July 17, 2016 - According to FAQ (RDS) there is no need to chose a specific AZ when purchasing reserved instances for RDS. Is this true? https://aws.amazon.com/rds/faqs/ "Q: Will there always be reservati...
Find elsewhere
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Cloudhealthtech
cloudhealthtech.com › blog › amazon-rds-reserved-instances
VMware Tanzu CloudHealth | VMware Tanzu
2 weeks ago - VMware Tanzu CloudHealth simplifies financial management, streamlines operations, and improves organizational collaboration across your multi-cloud environment.
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Amazon Web Services
docs.aws.amazon.com › amazon rds › user guide › what is amazon relational database service (amazon rds)? › db instance billing for amazon rds › reserved db instances for amazon rds
Reserved DB instances for Amazon RDS - Amazon Relational Database Service
If you have a DB instance, and you need to scale it to larger capacity, your reserved DB instance is automatically applied to your scaled DB instance. That is, your reserved DB instances are automatically applied across all DB instance class sizes. Size-flexible reserved DB instances are available for DB instances with the same AWS Region and database engine.
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AWS
docs.aws.amazon.com › amazon ec2 › user guide › amazon ec2 instances › amazon ec2 billing and purchasing options › reserved instances for amazon ec2 overview
Amazon RDS reserved DB instances
Reserved Instances are not physical instances, but rather a billing discount applied to the use of On-Demand Instances in your account. These On-Demand Instances must match certain attributes, such as instance type and Region, in order to benefit from the billing discount.
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AWS
aws.amazon.com › products › database › amazon rds
Managed Relational Database - Amazon RDS Pricing - Amazon Web Services
5 days ago - As a result, you’re freed from the complexity and costs of planning, purchasing, and maintaining hardware. This transforms what are commonly large fixed costs into smaller variable costs. ... With Reserved Instances, you can reserve a DB instance for a one- or three-year term.
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Trend Micro
cloudconformity.com › knowledge-base › aws › RDS › unused-reserved-instances.html
Unused RDS Reserved Instances | Trend Micro
November 8, 2022 - Because AWS RDS Standard Reserved are reserved for you at purchase, you cannot modify, cancel or sell these reservations and you will be billed for these resources regardless of whether you use them. To make use of your investment, you can provision and utilize a corresponding RDS DB instance ...
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Cloudability
support.cloudability.com › hc › en-us › articles › 115013610847-RDS-Reserved-Instances-Instance-Size-Flexibility
RDS Reserved Instances: Instance Size Flexibility – Documentation and Best Practices
April 19, 2021 - ISF is a feature of RDS RIs that lets the savings of a RI apply to any size of instance within a family. You automatically get this benefit when you purchase a RI within the same AWS Region (for both Single-AZ and Multi-AZ configurations), database ...
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Reddit
reddit.com › r/aws › rds reserved instances
r/aws on Reddit: RDS Reserved Instances
November 25, 2023 -

Haven’t bought these before and wondering if there are any catches I should be aware of. I fully expect to use all the resources I will purchase under a no upfront RI, the savings are 24% and there doesn’t seem to be any downside.

Am I missing something, seems too good to be true.

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ProsperOps
prosperops.com › home › amazon rds reserved instances: everything you need to know!
Amazon RDS Reserved Instances: Everything You Need to Know! - ProsperOps
July 30, 2024 - Amazon Relational Database Service (RDS) Reserved Instances (RIs) are long-term reservations that offer significant cost savings over On-Demand pricing in exchange for a commitment to specific database instances for a one- or three-year term.
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AWS re:Post
repost.aws › articles › ARd-aVHZYdQS25eN9GBlTypw › optimizing-reserved-instance-purchases-for-rds-when-instance-scheduling-is-in-use
Optimizing Reserved Instance Purchases for RDS when Instance Scheduling is in use | AWS re:Post
October 4, 2023 - Amongst the various commitment based discount schemes offered by AWS, two of the most commonly used are Savings Plans (SP’s) and Reserved Instances (RI’s). Savings Plans offer a great deal of flexibility - however they can only be purchased for certain resource types. One of the resources that Savings Plans cannot currently be used against are RDS instances.
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Easydeploy
easydeploy.io › home › how to purchase aws reserved instance for rds
How to Purchase AWS Reserved instance for RDS
January 11, 2024 - Select your AWS RDS engine type and instance class type carefully. Then under “deployment option” select your RDS AZ either single AZ or multi-AZ · Select your required term as per your convenience from 1 year to 3 years · Select the “offering type” all upfront, partial upfront, or no upfront based on your requirement. Then Enter the Name for your reserved instance, for better identification you can give the same name as your AWS RDS instance.
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Trend Micro
trendmicro.com › cloudoneconformity › knowledge-base › aws › RDS › reserved-instance-purchase-recommendations.html
AWS RDS Reserved Instances Purchase Recommendations | Trend Micro
October 30, 2020 - If the selected RDS instance is not the component of an application stack that is about to be decommissioned and the database will be in use for at least 6 months, the instance is stable and qualifies as candidate for the upgrade to a Reserved Instance (RI). To upgrade it you must follow the recommendations outlined in the Remediation/Resolution section and purchase an RDS RI based on the active (On-Demand) instance attributes. 07 Repeat step no. 4 and 5 to determine if other RDS database instances available in the current region are eligible for the upgrade. 08 Change the AWS region from the navigation bar and repeat the process for the other regions.