As with the PC game the players available within a game are biased somewhat by the nations you select at the start of the game.
The 'rough' way FMH/FMHi operate is that there is a near enough 'static' number of players in the game (it varies a 'bit' by +/- 10% or so) - these distribute in this manner:
<Core Players> (players famous enough to be in ALL games)
<Players from selected leagues> (players associated with selected leagues)
The 'Core Players' are always present and don't really change, however for selected leagues the amount 'active' is somewhat affected by the number of leagues running - hence when running multiple leagues squads might be 'light' a few players who could be present if you were just running a single league.
(hope that made sense)