You determine the size of the bonus as the melee ability resolves. Count each opponent that you attacked with one or more creatures. It doesn't matter if the attacking creatures are still attacking or even if they are still on the battlefield. It also doesn't matter if the opponent you attacked is still in the game.

It doesn't matter how many creatures you attacked a player with, only that you attacked a player with at least one creature. For example, if you attack one player with Wings of the Guard and another player with five creatures, Wings of the Guard will get +2/+2 until end of turn.

Melee will trigger if the creature with melee attacks a planeswalker. However, the effect counts only opponents (and not planeswalkers) that you attacked with a creature when determining the bonus.

Creatures that enter the battlefield attacking were never declared as attackers, so they won't count toward melee's effect. Similarly, if a creature with melee enters the battlefield attacking, melee won't trigger.