I think it depends how well they play, the most i had in my team was three, Kivi (CF)and Piliniak (CF) and Hugo (DMF). Kivi and Piliniak improved hugely because they scored goals but with Hugo he never improved because I didn't know how to play a DMF well. He also stopped being promising first.
With strikers it was easy, all they have to is score as many goals as possible with minimum shots taken, but others it is harder.
Goalkeepers might sound easy, but you have to make them save as well as keeping a clean sheet, or they will just get ratings of about 6.0 and never really improve.
My other piece of advice would be to have one promising player at a time, so you can concentrate on getting them to be good, like passing all the time to them so they can shine.
The most points I have gotten from a game to improve players is 90 for Piliniak.