Let me talk about TV, because I love TV. Even though it can be frustrating - things get stuffed in episodes that aren't long enough and some topics never get addressed. Sometimes that makes me wonder how stupid the writers are or how stupid they think the viewers are. Sometimes I learn that the writers did think about those things or even put those things in, but then they were cut. ...I like to think that in a parallel universe they were left in, haha.

So!

I love the idea of multiple universes, the idea of every decision that is made creating an alternate universe and of people travelling between these worlds. But there's something I think I haven't seen mentioned on any show yet. What happens when people travel to an alternate universe and after that there are still new universes being created from their original one or the one they're in at the moment?

When the original universe is branching off while the travelers are away it would mean they have several worlds to come home to. They could choose, if they wanted. So some worlds would never get their travelers back. That's assuming that the travelers aren't affected by the split.
But this is where it starts to hurt my brain. Maybe there's a connection between the universe and everything that was created in it, so that everything is branching off, whether it's in its own universe or not. What that means for the travelers in the alternate universe, I don't know. Does the second set of travelers just appear in the same universe where the travelers currently are? Since decisions are being made constantly that would be a real mess. See, that's what makes my brain hurt. Maybe it just results in a split of the alternate universe.

Or the alternate universe is branching off (well, both will be branching off, but it's likely one will more that the other) and there are more travelers than original worlds and some won't be able to go home. Unless people are nice enough to accept the 'copies' there.

Hm, I swear I had more thoughts on that, but now I can't remember. ...Oh well, I'm sure in some parallel world I did remember.