setting and adjusting measures per staff

Format/Add/Remove System Breaks

But also, note you normally don't need to do this - MuseScore will expand measures as necessary when you start adding notes to your score. Or, if you are starting with the default demo score, it has system breaks already, you'd need to delete those. But don't start off assuming you need to set a fixed number of measures per line, that's rarely the right thing. Normally you'd make adjustments only when done entering music, and it would be done system by system.