First for us it depends on how many are worth running on from the puppy eval. One litter we only ran on one pup. Then another we ran on three. I guess it depends also on how much room you have.
It's according to the litter, and what I am looking for. I have run on as many as 4 untill the 6 month mark, then dropped down to 3.
I usually will keep two , in case one "goes south" for some reason or another, when testing starts , that usually will eliminate someone.
I usually keep two. I also, find three harder to keep and socialize. More likely to have accidents when playing with three. Can work for a few months, but 6 mos old are big pups to hit each other running and playing. Once in a while one pup.