Rulings

• 2019-06-14
  Protection from Humans means that Yawgmoth can’t be blocked by Human creatures, can’t be the target of Human spells or abilities from Human sources, and all damage that Humans would deal to it is prevented. It also means that Yawgmoth can’t be enchanted or equipped by an Aura or Equipment that is somehow a Human, although this can only occur with certain combinations of older cards.
• 2019-06-14
  You can choose any permanent that has a counter, including ones controlled by opponents. You can’t choose cards in any zone other than the battlefield, even if they have counters on them (such as suspended cards).
• 2019-06-14
  Because proliferate doesn’t target, you can proliferate counters on Yawgmoth with its own ability if it gets a counter.
• 2019-06-14
  You don’t have to choose every permanent or player that has a counter, only the ones you want to give another counter. 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.
• 2019-06-14
  Players can respond to the 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.
• 2019-06-14
  You may activate Yawgmoth’s first activated ability without choosing a target creature. You’ll just draw a card. However, if you choose a target and it becomes illegal before the ability tries to resolve, the ability won’t resolve and you won’t draw a card.
• 2019-06-14
  The Modern Horizons set doesn’t include any ways for players to have counters to proliferate, but some cards in other sets say that a player “gets” a counter of a certain kind. Notably, emblems aren’t counters.
• 2019-06-14
  Protection from Humans refers only to the creature type Human. As far as Yawgmoth is concerned, you and your opponents aren’t Humans.
• 2021-03-19
  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.
• 2021-03-19
  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.
• 2021-03-19
  If a permanent has +1/+1 counters and -1/-1 counters on it, they’re removed in pairs as a state-based action so that the permanent has only one of those kinds of counters on it.
• 2021-03-19
  To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can’t choose cards in any zone other than the battlefield, even if they have counters on them.
• 2021-03-19
  Protection from Humans refers only to the creature type Human. As far as Yawgmoth is concerned, you, your opponents, and planeswalkers aren’t Humans.