No more than one creature can attack each combat.
No more than one creature can block each combat.
Whenever you roll, Astral Arena deals 2 damage to each creature.
6/1/2012 Astral Arena’s abilities stop more than one creature from being declared as an attacker or as a blocker each combat. However, it doesn’t affect creatures that are put onto the battlefield attacking or blocking.
6/1/2012 If a turn has multiple combat phases, either a different creature or the same one can attack or block in each of them.