measures per line

MuseScore fits as many measures as between the page margins as it can before starting a new line (or system as MuseScore calls them) - just like a word processor app will fit as many words on a line as it can and longer words mean you can fit fewer of them between the margins. To get fewer measures add a system break where you want a system to end (as you would add a return in a word processor to start a new line. To get more measures than will currently fit between the margins you will need to use a large page size, reduce the margin size, reduce the staff space scaling (make the notation smaller (like using a smaller font in a word processor) or a combination of some or all of those.

See https://musescore.org/en/handbook/3/breaks-and-spacers
and See https://musescore.org/en/handbook/3/page-settings

Format >Add/Remove System Breaks >Break systems every x bars.