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, Team Avatar's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

Similarly, the damage dealt by Team Avatar's activated ability is calculated during the resolution of the ability.

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