Players loading at the begining of the game

when starting a new game, does loading around 50,000 players keep 50,000 players in the game for as long as you are playing? I mean do regens replace retired players or will the player count eventually go down if i keep playing and dont load up any more leagues?

No, it should stay around the same level. It will fluctuate a bit, but shouldn't be much more than 5,000 either way.

If you load and remove a lot of leagues then you might see bigger changes.