Different types of staff have different effects on your club:
For these staff members, the more you hire, the less effective the additional staff will be. (ie the second coach hired will not add as much benefit as the first, nor the third as much as the second, and so on.)
To optimise training outcomes, 10 psychs are "ideal" - but you'd need an electron microscope to spot the difference. It all depends how much money you want to pay per week to gain that miniscule advantage.
Lets say the reduction in effectiveness for subsequent staff is 50%:
- 1st net: 100% gain
- 2nd net 50% gain
- 3rd net 25% gain
- 4th net 12.5% gain
- 5th net 6.75% gain
- 6th net 3.125% gain
- 7th net 1.5625% gain
That 1st net is having 64 times more effect than the 7th net. Even if a player is trained all the way to elite conc/cons from mediocre (16 levels) that 7th net will be responsible for less than 13% of one level difference.