Does Jinx Rebel Stack Riftbound?

Yes, Jinx Rebel's ability stacks — but only per discard event, not per card.

Each time you discard one or more cards as a single event (e.g. paying a cost, resolving a spell), she triggers once: ready her and give her +1 Might for the turn. If you trigger her three separate times in the same turn (e.g. via three Super Mega Death Rockets resolving individually), she gets +3 Might and readies three times. (FAQ #1411, #5434)

However, if you discard multiple cards at once (e.g. Invert Timelines discarding your whole hand), that's one event — she triggers exactly once, getting +1 Might, not +4. (FAQ #11654)

Bottom line: number of discard events = number of triggers. A single event that discards many cards still only gives +1.