Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
If a permanent ever has both +1/+1 counters and -1/-1 counters on it at the same time, they're removed in pairs as a state-based action so that the permanent has only one of those kinds of counters on it.
If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
This printing is one of 3 catalogued treatments from PRM — the lineup includes Foil and Nonfoil alongside the standard finish. Across our catalog the Rarix library tracks every variant in the same family so collectors can compare price, scarcity and demand inside a single lineage.
Card #82874 of *Magic Online Promos*, the promotional run that shipped alongside the August 2020 Magic Online release.
Card type: Legendary Creature — Phyrexian Angel Horror, color identity black/green/blue/white, printed at mythic. Mythic rares historically land in roughly one out of every eight boosters at standard print densities. Mana cost: {G}{W}{U}{B} (4 converted). Keyword abilities: Deathtouch, Flying, Lifelink, Vigilance, and Proliferate.
Legal in Legacy, Vintage, Commander, Brawl, and Historic.
EDHrec ranks this card #2,429 in Commander, putting it in the niche-pick tier of the format's playable pool.
Illustrated by Kev Walker. Rarix catalogs 1,917 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Kev Walker
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
You don't have to choose every permanent or player that has a counter, only the ones you want to add another counter to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
An ability that triggers "Whenever you proliferate" triggers even if you chose no permanents or players while doing so.
To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.
