I don't really care about the numbers of players allowed to be loan in or out. So I vote for 3. But I hope player loan will not be used as a method to insider dealing by using a team with more resources to support another team. So I suggest:

1) adequate numbers of tokens should be consumed on loan, probably by both parties
2) there is a reasonable loan fee which should not be deviated far from one third of the buying price or market price of the player for 1 season loan
3) the loan should be between clubs in same league level

Actually, I don't think it is not easy to make this arrangements fair.