Exiling the card with encore is a cost to activate the ability. Once you announce that you're activating it, no player may take actions until you've finished. They can't try to remove the card from your graveyard to stop you from paying the cost.

If the creature card has in its mana cost, you can choose any value for X, but the value you choose won't apply to the tokens that are created. X will be 0 for those tokens.

If one of the tokens can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, you aren't forced to pay that cost, so it doesn't have to attack in that case either.

Each token must attack the appropriate player if able.

If one of the tokens somehow is under another player's control as the delayed triggered ability resolves, you can't sacrifice that token. It remains on the battlefield indefinitely, even if you regain control of it later.

The tokens copy only what's on the original card. Effects that modified that creature when it was previously on the battlefield won't be copied.

Opponents who have left the game aren't counted when determining how many tokens to create.

If an effect stops a token from attacking a specific player, that token can attack any player, planeswalker, or battle, or not attack at all. If the effect stops the token from attacking a specific player unless a cost is paid, you don't have to pay that cost unless you want to attack that player.