On the internet, you will see a lot of advice about "if you allocate too much memory, the garbage collector will let all the memory fill up, then create a massive 'gc pause' lag spike as it clears out all the memory!". This used to be true in the Java 8 days using the "Parallel" or "ConcurrentMarkSweep" collectors, which were prone to doing that unless you used a massive wall of Java arguments. But for years now, the default garbage collector is G1, which optimizes for minimal pause times right out of the box. G1 is a very smart collector and it will likely do what you want without having to think about it too much.

Aikar's flags look good but I would be skeptical about those "here, paste these 5 million java arguments" lists.

That said, there is an important JVM memory optimization called "compressed oops" that is only possible to leverage if you allocate less than 32gb of memory. If you allocate more than 32gb, every single object on the Java heap becomes much larger, so you will consume memory faster than if you allocate less. Never allocate more than 32gb of RAM to Java (unless you are allocating way more than 32gb, like 100gb)

Also, set the minimum RAM and maximum RAM to the same value. If there's wiggle room, the JVM is allowed to release memory it's not using at the moment back to the computer, and then it will have to waste time reclaiming it again when it needs more memory.

tl;dr 16gb