In 4 players FFA How meny players can be at the same battlefield while showdown? And how to deal with cards that can create this situation like moonfall and Irresistible Faefolk

Max is 2 players — combat (and any showdown) only ever involves exactly two players' units in FFA4 (CR 464).

A third player can never have units at the battlefield where a showdown/combat is occurring. That battlefield is an Invalid Destination for their units' moves (CR 449.2.a, 464.1), they can't play units into it (CR 464.2 — they'd go to Base instead), and no unit may move to a battlefield already holding two other players' units "by any means" (CR 451.2). So while other players can cast spells during your showdown (turn order from the attacker), their units cannot be there.

Moonfall and Irresistible Faefolk cannot force a third player into an existing fight. The enemy unit they move is subject to the same destination rules: if it's controlled by a player not involved in the combat who has no units there, the destination is invalid, and the choice itself is "invalid and ineligible to be completed" (CR 464.3). Since both effects are optional ("you may"), you simply move nothing. You can only use them to (a) pull a unit from an enemy who already has units at that battlefield (that unit just becomes the defender), or (b) start a fresh 2-player fight at a battlefield where only you have units — e.g., during someone else's showdown, target a different battlefield you hold; that new fight stages and waits for the current one to finish (FAQ #5108). Moonfall's -2 penalty still hits enemy units there even if you move no one.