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.

Tifa's last ability doesn't give you any additional main phases. This means that you will move directly from the end of combat step of one combat phase to the beginning of combat step of the next one.

Melee will trigger if the creature with melee attacks a planeswalker or battle. However, the effect counts only opponents (and not planeswalkers or battles) 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.

Tifa's last ability cares about the power of each individual creature that dealt combat damage to a player, not their total power.

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 Tifa and another player with five creatures, Tifa will get +2/+2 until end of turn.