if its a good strain and grown well, just one wil do.

if you mean selecting the best/strongest individuals for long term cloning or breeding, well you cant really put a number on it, theroreticly you could grow out seeds forever to find the strongest plants.

you might get a runt once in a while but reputable breeders strains are generally good, because a lot of breeding work has already been done to produce the seeds. its just the genetic wheel of fortune