The number of publish instances you need depend on a number of factors, which cannot be quantified in a general way. There cannot be any standardized performance test to tell you how much publish instances you need.
My recipee is to design a loadtest which is realistic to your scenario (typically you don't have the application yet which you need to operate). Then validate how much load a single publish instance can handle and do the math to calculate how much publishs you need to handle your anticipated load.
Jörg
