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

Count the number of creatures you control other than the equipped creature as Sigil of Valor's ability resolves to determine the amount of the bonus. Once the ability resolves, the bonus won't change, even if the number of creatures you control does.