I would stick to 2-3 main trainees to start with. It takes 3 seasons to train 3 players that can even start in Div 4. After you have those guys in the early 20's, it can be reasonable to add additional trainees to the core. If you are doing a homegrown setup for your main trainees, this will likely be the case as your guys will age and get closer to capping at varying rates. Once you are to the point where you have 6 decent trainees, you can look at 2 or 3 pos training as a natural strategy. Optimally you will have some guys who are starters and other guys who pretty much serve as backups/scrimmage guys. Once you get players close to capping, you can add more trainees and alternate training so that the guys that are close to capping just train in secondaries. After most of your team is close to capping, its time to consider just doing team training. At that point you will have a full roster (hopefully) ages 21-28 with 5 or so homegrown and 5 or so purchased without being capped. Unfortunately, the younger players may never reach potential for the sake of the team at that point so you may have to sell them.
That is what I hope to get to Div I with. However, young/uncapped/well trained players are very expensive. It will take good economic management and lucky bidding to get an ideal situation where at least 8 players are uncapped but good enough to play in Div I or II.