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 OverflowRDS Reserved Instances
amazon web services - Reserved instance limit for EC2 and RDS - Stack Overflow
RDS Reserved Instances
Changing class of a reserved instance in RDS from m3 to m5?
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Reserved instances are a billing construct only.
If you buy a reserved instance for a m1.small in us-west-2a, all that means is that one of your on-demand RDS instances that's an m1.small in us-west-2a receives that lower hourly price (and incidentally, if you delete that instance and make a new one, the reservation's lower hourly rate just shifts onto the new instance). No instance is launched by reserving one.
Think of it as purchasing a licence for one of your existing instances you already have rather than creating a new instance.
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.