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 $42.82. Over the trailing 90 days the price moved between $0.02 and $43.38 across 55 observations. Cross-checked across TCGplayer, eBay sold, Cardmarket EU, and Star City.
This printing is one of 3 catalogued treatments from MH1 — 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 #229 of *Modern Horizons*, the set that shipped alongside the June 2019 Modern Horizons release.
Card type: Artifact — Equipment, printed at mythic. Mythic rares historically land in roughly one out of every eight boosters at standard print densities. Mana cost: {3} (3 converted). Keyword abilities: Proliferate and Equip.
Legal across the major constructed formats — Modern, Legacy, Vintage, and Commander — plus Standard rotation play.
EDHrec ranks this card #1,653 in Commander, putting it in the common-include tier of the format's playable pool.
Illustrated by Chris Rahn. Rarix catalogs 1,209 other cards credited to this artist.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Chris Rahn
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
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.
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.
If a permanent or player you choose while proliferating has more than one kind of counter on it, it will get one more of each of those kinds. This means that if you put a +1/+1 counter on a creature you control with a -1/-1 counter on it, then choose that creature while proliferating, you'll put another of each of those counters on it before state-based actions remove them.
Protection from a color means that the equipped creature can't be blocked by creatures of that color, can't be the target of spells of that color or abilities from sources of that color, can't be enchanted or equipped by Auras or Equipment of that color, and all damage that sources of that color would deal to it is prevented. Nothing other than these events is prevented or illegal.
