If Flurry of Wings is cast before attackers have been declared or after combat ends, it won’t do anything.

Flurry of Wings counts the number of creatures that are attacking as it resolves. It doesn’t matter how many creatures were originally declared as attackers that turn. It also doesn’t matter who controls those creatures, or who or what they’re attacking.

An “attacking creature” is one that has been declared as an attacker this combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left combat.