*sigh* you are trying to make some noob moves man

(no offense), but I got your back. ;) You should never get more than 3 trainees so you could sell some of your team to make room for three 18 year olds, but you should not get any more than three. This is because with two games a week along with a scrimmage you only have enough minutes to fully train three players. If you were to have 8, then none of them would come anywhere close to their potential. As for the trainer level, yes, 6 is probably too much. A level 4 trainer is perfect because there isn't that much of a difference between a level 4 and a level 6 trainer and a level 4 trainer would be much cheaper. If I were you this is the route I would take:
Buy two trainees (Either two guards together, two forwards together, or two big men together). I would look at 18 year olds with decent starting skills with at least P. Allstar potential. That is enough potential to get you two 50k+ players that you can train however you want and then you have an extra game per week to make sure they both get their minutes for training.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs