"Exalted (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.)"
This is a triggered ability that triggers for each instance of Exalted you have on the field. For example, if I have 4 permanents with Exalted and attack with one creature, that creature will get +4/+4.
Exalted ONLY triggers when you declare one and only one creature as an attacker. If you declare an attack with two creatures and one is destroyed or otherwise removed from combat, then the remaining creature will not get Exalted bonuses.