The only way this would work is if the time travel is such that what they do when they go back already happened; it isn't sustainable if the time travelers can change history.
Humanity at the end of the universe is theoretically a very large population, saturating the available space on inhabitable planets in the universe. Even if they could send themselves back to all the other planets besides earth (and assuming they were originally uninhabited), this would only work once. When you get to the end of time the second round, there's nowhere to go.
If the group becomes extinct before humanity emerges as a species, it works. If they integrate themselves into their own history (as in this Star Trek episode), it works. But either way, it only works once.
If you are dead set on making it possible, then humanity must have the capability to create a brand new universe each time they hit the reset button. If they don't, even if they can create new planets out of thin air and limit the number that goes back each time, they will eventually run out of room. Space is immensely vast, but ultimately finite. But at this point, time travel is arbitrary in making things work. They don't need to travel back in time unless they arbitrarily want all the universes running in sync together. And if they can create new universes to live in, they will do it well before the one they're living in runs out.