A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Asari Captain's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

Asari Captain's triggered ability gives the solitary attacker +1/+0 for each creature you control that is either a Samurai or a Warrior as it resolves, including the attacker itself, if applicable. A creature that is both a Samurai and a Warrior is only counted once.