If you power train, it is only worth training those who you want to stay for a long time. I'd say between 7-11 players. Enough for a full team.
50 packs isn't enough... 500 packs can get you a long way. It has been proven to be better than the personal trainer. If you buy scouts then buy one close to 7*... then you could power train him to 8* or even 9*. Just remember to buy one aged 22-24