Hello :)

Alliances are restricted in such a way you can never have more than 50% of the total numbers of players in your Alliance (the exception being 3 players when you cann ally in a 2v1, I think).

That means that if you have 5 players total (including yourself) - you can ally with a 2nd player but a 3rd would put you over the threshold of 50% (since you can't cut a player in half to ally with 2.5 players... Except if you brutally murder them which would drastically reduce their ability to play :D)

This is not a bug - but I agree a line in a tooltip wouldn't hurt :)

Thanks for your involvement.