If multiple players want to take the monarch can they all attack the monarch player in the same combat?

In a multiplayer game using the free-for-all format (or similar), each player who is taking a turn attacks during their own combat phase. However, the question likely refers to a single player's turn — in that case, only the active player attacks, so only one player can attack per combat.

But if you're asking whether, across a full round of turns, multiple players can each attack the monarch on their respective turns — absolutely yes. Each player, on their own turn, may declare the monarch as their attack target, and if they deal combat damage, they become the monarch (CR 716.3). The monarch status changes hands each time.

Within a single combat, only the active player declares attackers (CR 508.1). In certain multiplayer variants, some effects can allow additional players to attack during the same combat, but under normal rules, only one player attacks per combat phase.

Concrete example: It's a four-player game. Player A is the monarch. On Player B's turn, B attacks A with a 2/2, deals damage, and becomes the new monarch. Then on Player C's turn, C also attacks the now-monarch B with a 3/3, deals damage, and C becomes the monarch. Each did so in their own separate combat phase.

So multiple players CAN each try to take the monarch, just on their own respective turns — not simultaneously in the same combat step, unless a special effect grants additional attack steps to other players in the same turn.