Can i use trigger ability's multiple times? Like Fiora worthy

Yes — triggered abilities have no once-per-turn limit unless the card says so. Fiora, Worthy just repeats: "When a unit you control becomes [Mighty], you may pay [C] to ready it" — every time a unit makes that transition (Might going from under 5 to 5+), a fresh instance goes on the chain with its own [C] payment.

Note you're not "activating" it on demand — you only get a trigger each time the condition is actually met. If a unit drops below Mighty and then climbs back (spending a buff and regaining it, for example), that's a new transition and Fiora triggers again; same if several units become Mighty at once, in which case you get one instance per unit and choose their order (Rule 383.3.d).

The ability also won't trigger off a unit that enters play already at 5+ Might — there has to be a real change while it's in play. Limits like "once each turn" or "the first time" only apply when the text includes them (Rule 383.3.e), and Fiora, Worthy has no such wording.