Even long rests (10-20 min) won’t suffice to totally remove lactic acid from our muscles. Therefore, potential intensity of each following set will decrease. Experiments showed that developed strength drastically decreases after 5-6 sets (this is the average). As result, this number probably should be treated as maximal number of sets per muscle group, when your purpose is micro injuries[^1] (main component of muscle strength and mass). Note, that I told:”maximal number” since your purpose isn’t to cause as many micro injuries as possible, but an optimal number of them (recovering recourses might not be able to handle with too many micro injuries). It’s impossible to state specific amount of sets needed for optimal amount of micro injuries, since it depends on other factors (such as workout intensity and training level). But if I had to give a simple rule of thumb than it would be: when your goal is micro injuries than 1-6 sets per muscle group should be your choice.

[^1]For more information see: muscle damage theory.