How to set number of measures per system? How many systems per page?

"How to set number of measures per system? "
Menu path: Format > Add/remove system breaks > Break systems every "4" measures

"How many systems per page?"
You have to do this manually.
Select the system break after which you want to start a new page, and press Ctrl+Enter.
Or select the last note (or rest) of the measure, then Ctrl+Enter. (I think Cmd+Enter on a Mac?)

See Handbook about "Page Layout Concepts":
https://musescore.org/en/handbook/4/page-layout-concepts
... and about "Formatting":
https://musescore.org/en/handbook/4/formatting