How do I control the number of measures in staves?

Add a system break to force the next measure onto the next system. Fiddle with margins, paper size, staff scaling and stretch if you want to fit more measures on a system than currently fit. Don't do any of that until you have entered the music as Musescore can generally make a good job of laying out measures on the page with its default settings.

By adding system breaks you can restrict the number of measures in a staff, by reducing stretch you can extend it (to a limit)