If I buy this do I get updates until the next Engine number jump or for life? How does does that work?

Yes! By buying it right now, you'll be guaranteed to get all 1.x.x updates, meaning that you won't have to worry about that for many years to come.

As a reminder, after 1.9.9 it's not necessarily 2.0, it follows itch's version control convention: major.minor.patch, so something like 1.999999999999999.999999999999999 is completely possible. 😅