Adriana, Captain of the Guard

Mana cost: {3}{R}{W}
Type: Legendary Creature — Human Knight
Power / toughness: 4 / 4
Set: Commander 2020 #200 (rare)
Price: $0.41 USD

Melee (Whenever this creature attacks, it gets +1/+1 until end of turn for each opponent you attacked this combat.)
Other creatures you control have melee. (If a creature has multiple instances of melee, each triggers separately.)

Adriana, Captain of the Guard has 5 printings, first in Conspiracy: Take the Crown (2016). EDHREC ranks it #4,971 among Commander cards. It appears most in Isshin, Two Heavens as One decks (6,674 on EDHREC). 3 public decks on TCG run it. It has 4 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: not legal
pioneer: not legal
standard: not legal
brawl: not legal

Public decks that run Adriana, Captain of the Guard

Legends' Legacy - Dominaria United, led by Dihada, Binder of Wills
Rebellion Rising - Phyrexia: All Will Be One Commander, led by Neyali, Suns' Vanguard
Ruthless Regiment - Commander 2020, led by Jirina Kudro

Commanders that run Adriana, Captain of the Guard most

Deck counts from EDHREC.

Often played with Adriana, Captain of the Guard

Cards that show up in the same decks on EDHREC.

Rulings

2016-08-23: 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.
2016-08-23: 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.
2016-08-23: 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.
2016-08-23: 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.