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, exalted won't trigger if you attack with multiple creatures and all but one of them are removed from combat. Similarly, creatures that enter the battlefield attacking later in combat won't affect exalted abilities that have already triggered or resolved.

A creature with multiple exalted counters will have that many instances of exalted.