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Am I paying for one version or for every version to come?

i think every to come

As getjinxed mentioned when you pay once on Itch.io you should have access to any new versions/updates on itch

I've seen games where the owners claim to charge money for every new version instead of paying once

Those are likely the devs that sell an early version of their game and then "forget" to update it as they continue to work on it and give it out on Patreon for paying customers.