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.
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.
Last observed on Scryfall 2 months ago at $0.75. Over the trailing 90 days the price moved between $0.03 and $0.75 across 46 observations. Cross-checked across TCGplayer, eBay sold, Cardmarket EU, and Star City.
This printing is one of 3 catalogued treatments from MM2 — 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 #84 of *Modern Masters 2015*, the set that shipped alongside the May 2015 Modern Masters 2015 release.
Card type: Instant, color identity black, printed at common. Commons fill the bulk of a booster pack's volume. Mana cost: {2}{B} (3 converted). Keyword abilities: Proliferate.
Legal in Modern, Legacy, Vintage, Commander, and Pauper.
EDHrec ranks this card #4,540 in Commander, putting it in the niche-pick tier of the format's playable pool.
Illustrated by Erica Yang. Rarix catalogs 147 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Erica Yang
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
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.
You may choose the creature you put the -1/-1 counter on to get another -1/-1 counter when proliferating.
