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.
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.
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.
Last observed on Scryfall 2 months ago at $1.19. Over the trailing 90 days the price moved between $0.03 and $1.21 across 55 observations. Cross-checked across TCGplayer, eBay sold, Cardmarket EU, and Star City.
This printing is one of 3 catalogued treatments from WAR — 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 #205 of *War of the Spark*, the set that shipped alongside the May 2019 War of the Spark release.
Card type: Creature — Merfolk Mutant, color identity green/blue, printed at uncommon. Uncommons fill the middle-tier slots in a standard booster configuration. Mana cost: {G}{U} (2 converted). Keyword abilities: Flying and Proliferate.
Legal across the major constructed formats — Modern, Legacy, Vintage, and Commander — plus Standard rotation play.
EDHrec ranks this card #6,307 in Commander, putting it in the niche-pick tier of the format's playable pool.
Illustrated by Sara Winters. Rarix catalogs 179 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Sara Winters
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
An ability that triggers "Whenever you proliferate" triggers even if you chose no permanents or players while doing so.
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.
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.
You may choose Merfolk Skydiver as the target of its own ability.
