I'm aware of that. I mean that there are fewer effectively different worlds than that. Like, if you consider a scattering event - a major source of decoherence and thus new world formation.

With which other particle will this particle's next interaction (over some minimal threshold of impulse) occur, cut off by time X in some reference frame (e.g. our frame, right now)? (setting aside the fact that particles don't have individual identities, since that would decrease the world count)

This is a stronger question than that asked by decoherence, but you can already see we've narrowed it down to something that seems like picking out of a list - even if the universe is infinite, the number of particles in a truncated future light cone is finite.

Of course you're doing this a huge number of times, and each one creates a huge number of alternatives, but that's just making an unimaginably huge finite number. See?