I found a lot about bar numbers, but not where to turn them on or off. Help appreciated
In the top menu: “View > Bar numbers > Page view/Galley view” (it might read something different, I don’t have English activated right now, but hopefully it’s clear). Is that what you mean?
If you are instead talking about how to make bar numbers appear in the music itself for printing in the score and parts, see the Bar Numbers page of Layout Options.