Training one position? In the example below we're training player 1 at PG, each number corresponds to a player, set as starter / backup / reserve:
1 1 1
2 6 6
3 7 7
4 8 8
5 9 9
You can go 2 6 7, 3 7 6, etc. and be fine as well, as long as you dress less than 9, the trainee is set in all 3 roles for his position, and each non starting player has a backup role they should be fine.
Fine most of the time anyway, if a player has low stamina they'll sometimes come out at 45 mins+ (and sometimes go back in again) - if you know your trainee has low stamina play a slow tactic
Foul outs can be still be an issue obviously - I try to make sure they guard either pg/sg, (or sf if they have some inside defence/reb and its not a dominant inside player they'll face), most fouls coming defending inside shots...
If you're training 2 positions you need to dress a maximum of 8 players, and go 1 1 1, 2 2 2, instead of the above, but same theory + backup/reserve patterns.