Does extort trigger once per spell cast or once per creature with extort you control?

Extort is a triggered ability that reads: "Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life." Crucially, each instance of extort on each permanent you control is a separate ability that triggers independently whenever you cast a spell (CR 702.101a).

This means if you control three permanents each with extort, casting a single spell will cause three separate extort triggers to go on the stack. You may pay {W/B} for each trigger individually, potentially draining each opponent for up to 3 life total and gaining 3 life yourself.

Each trigger resolves one at a time, and you make the payment decision for each trigger separately as it resolves (CR 702.101b). You are never forced to pay for any of them — each is optional.

Concrete example: You control Syndic of Tithes and Crypt Ghast, both of which have extort. You cast Murder. Two extort triggers go on the stack. You pay {W/B} for the first trigger: each opponent loses 1 life, you gain 1 life. You then pay {W/B} for the second: each opponent loses another 1 life, you gain another 1 life. Total drain: 2 life per opponent, 2 life gained.

This is governed by CR 702.101a and the general rule that multiple instances of triggered abilities each trigger separately (CR 112.9).