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.
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.
Last observed on Scryfall 2 months ago at $5.71. Over the trailing 90 days the price moved between $0.03 and $6.31 across 58 observations. Cross-checked across TCGplayer, eBay sold, Cardmarket EU, and Star City.
This printing is one of 3 catalogued treatments from MBS — 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 #103 of *Mirrodin Besieged*, the set that shipped alongside the February 2011 Mirrodin Besieged release.
Card type: Artifact Creature — Phyrexian Horror, printed at uncommon. Uncommons fill the middle-tier slots in a standard booster configuration. Mana cost: {4} (4 converted). Keyword abilities: Proliferate and Infect.
Legal in Modern, Legacy, Vintage, and Commander.
EDHrec ranks this card #7,911 in Commander, putting it in the niche-pick tier of the format's playable pool.
Illustrated by Dave Allsop. Rarix catalogs 258 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Dave Allsop
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.
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.
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.
