Rulings

• 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
  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
  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.
• 2019-06-14
  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.
• 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
  You choose a creature, put a +1/+1 counter on it, then proliferate all while the triggered ability is resolving. Nothing can happen between these things, and no player may choose to take actions.