The value of X is determined only as Angelic Exaltation’s ability begins to resolve. It won’t change later in the turn if the number of creatures you control changes.

A creature attacks alone if it’s the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, Angelic Exaltation’s ability won’t trigger if you attack with multiple creatures and all but one of them are removed from combat.