Tromell, Seymour's Butler

If you proliferate multiple times, you don't have to choose the same set of players and/or permanents to get additional counters each time.

The value of X is determined only once, as Tromell's last ability resolves. To determine that value, look at the nontoken creatures you control and count each one that entered this turn. It doesn't matter if they were creatures when they entered or who controlled them when they entered.

When you 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.