Measures

Use the menu Edit->Tools->Add/Remove Line Breaks...

To expand on that:

MuseScore uses standard engraving practice, which is to space notes evenly and relatively tightly and put as many measures per line as fit. Normally you don't mess with that, the measures will adjust in width automatically and correctly as you enter notes. Only in the special situations where you deliberately want to create a consistent number of measures per line (eg, children's books, jazz lead sheets, a couple of others) would you normally want to mess with adding breaks to force this. But the above is how you do it. If you find a case where you want more measures on a line than fit by default, you'll instead need to squeeze the notes together - select and use Layout / Decrease Stretch, or lower the overall music size in Layout / Page Settings.