Mutational Advantage

The set of permanents affected by Mutational Advantage is determined at the time Mutational Advantage resolves. Permanents that gain counters later in the turn won’t become affected by this effect, and permanents that lose all of their counters later in the turn won’t stop being affected.

You don’t have to choose every permanent or player that has a counter—only the ones you want to add counters 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.

Some spells and abilities that cause you to proliferate may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won’t resolve. You won’t proliferate.