Can i use trigger ability's multiple times example sett brawler spend buff and got buff and spend again

Yes — and the Sett example works: his "Spend my buff: Give me +4 might" is an activated ability (cost: effect), so it can be activated any number of times per turn as long as you can pay the cost each time. Since only one buff can be on him at a time (Rule 702.3), you need a fresh buff source between activations — spend the buff, have Pit Rookie re-buff him, spend it again for another +4.

Remember his "when I'm played and when I conquer, buff me" is the triggered part, and that one is limited: it fires once per play and once per Conquer, and each battlefield can only be scored once per turn, so you can't re-farm it.

For triggers generally: they fire every time the condition is genuinely met unless the card says "once each turn" / "the first time…" — and Attack/Defend triggers only check once per combat (Rules 383.3.e, 383.4.e.2.a). Also note activated abilities like Sett's only work on your turn during an Open State (Rule 381).