i personally have 10 and if i need to buy more to fill my lineup for the scrimmage then i buy someone to replace one of my current top 10 (non-trainee) or a 1k player i can sack or try and re-sell.
the rules say 8 good players + 4 reserves (or something to that effect)
10 x 72mins in a perfect world should keep everyones gameshape high then if you get an inj you can dip into transfer market.... in the early days why spend loads on salaries on players that are just there to play half a scrimmage?
i train 4 x 18yo and then 2 from my other 4 main players depending on what im training that week.... SG/SF, SF/PF or PF/C i can just about get away with SG/PG as well and i feel this gives me as much flexibility i need....