From LOTR-TCG Wiki
From the Comprehensive Rules 4.0:
At the start of the regroup phase, for each of your characters who has the muster keyword, you may discard a card from hand, then draw a card. You must discard all at once.
If you have two muster characters, you cannot choose to discard a card, draw a card, then discard another card and draw a card. You must instead choose to discard 0, 1, or 2 cards, then draw that same number of cards.