How many running instances do you need though? e.g. Scala web frameworks should be able to do thousands of RPS on a single core without the application develope

How many running instances do you need though? e.g. Scala web frameworks should be able to do thousands of RPS on a single core without the application developer really trying to optimize anything, and I always hear that even Ruby, Python, etc. are also fast enough to be IO bound so you should just need 2 copies for redundancy, right? Then give each like 8-16 connections.