No more than one creature can attack each combat. No more than one creature can block each combat. Whenever chaos ensues, Astral Arena deals 2 damage to each creature.
2012-06-01
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.
2012-06-01
If a turn has multiple combat phases, either a different creature or the same one can attack or block in each of them.