A bugbear ranger 11/fighter 2 with a bow and a glaive as well as the polearm master feat and the tunnel fighter fighting style.
Ranger 11 - volley:
You can make a weapon attack against every creature within 10 feet of a point you can see.
This covers 9×9×9 tiny flying creatures =729 attacks.
While in your defensive stance, you can make opportunity attacks without using your reaction.
While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.
Your reach is 15 feet (+5 from bugbear, +5 from glaive) which covers 14×14×14-8 tiny flying creatures = 2736 opportunity attacks.
You can take 2 actions on the same turn
Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target.
You are flying and about to be surrounded on all sides by a horde of tiny flying creatures.
You then ready a volley (729) for when one of them gets too close.
You also have the tunnel fighter fighting style and use your bonus action to enter a defensive stance.
After firing you volley they all sound you triggering the 2736 opportunity attacks.
Now your turn begins, you take the volley action, then you take it again with action surge. That's another 729*2=1458 attacks.
Reaction volley - 729
Opportunity attacks - 2736
Action volley - 729
Action surge volley - 729
Horde Breaker - 1