Noble Hierarch

If you declare exactly one creature as an attacker, each exalted ability on each permanent you control (including, perhaps, the attacking creature itself) will trigger.

You must attack with exactly one creature for exalted abilities to trigger. Exalted abilities won't trigger if you attack a player with one creature and a planeswalker with another, for example, or if you attack with two creatures but one is removed from combat.

Some effects put creatures onto the battlefield attacking. Since those creatures were never declared as attackers, they're ignored by exalted abilities. They won't cause exalted abilities to trigger. If any exalted abilities have already triggered (because exactly one creature was declared as an attacker), those abilities will resolve as normal even though there may now be multiple attackers.

In a Two-Headed Giant game, a creature “attacks alone” if it's the only creature declared as an attacker by your entire team. If you control that attacking creature, your exalted abilities will trigger but your teammate's exalted abilities won't.