Agent Frank Horrigan

While proliferating twice, players can’t respond between proliferating the first time and proliferating the second time.

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.

The ability that causes Agent Frank Horrigan to have indestructible starts to apply as soon as it’s declared as an attacker. It applies for the remainder of the turn. It doesn’t matter what happens to the player or permanent it attacked after that point.

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.

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.

If Agent Frank Horrigan enters the battlefield attacking, it doesn’t have indestructible since it was never declared as an attacker.