You can train as many people as you want as long as they get at least 48 minutes of playing time (if they get less than 48, they'll still train, but at a lesser rate).
Also, if you train Passing with the PG/SG option, anyone who plays either position will be trained (but all at lower rate than just focusing on PG's).
Also remember that they're not going to pop a skill level every week, it takes 2-3 weeks depending on the players age. And if they're above the age of like 25, it takes a very long time to pop and really is not worth it from a training perspective.