Also just as an aside, its quite common for newer players to the game to want to train more players to begin with - I know that i certainly did this when i started.
But longer term, you want to have focused and deliberate training plan with longer term outcomes. That is - 3-4 seasons from now. Rather than just wanting 5 of your guys to get 1 or 2 pops for the remainder of the season.
There are several benefits of having longer term plans and focusing your training on 1-2 players. Single position will help them improve faster, thus giving you more value out of the training minutes.
But there are also thigns such as the fact that half way through the season, you are getting skills from players, which you aren't paying in salary yet. (Because salary only updates at end of season). your player which had 'Strong' ID, half waythrough the season has Prominent or Prolific ID, but his salary hasn't updated yet. So this is a bonus.
You also are building players that are more valuable when you sell them. in the longer term, thats also a good thing.
So yer, Mickyster has given some other info about why its good to train single position as well, whihc is valuable info.