how to add another page

MuseScore will flow over to a new page if and when needed. You can force it to so so early by using a page break

Since the newly added measures are currently empty, they don't take much room. But as you add notes, the measures get wider, and the pages take care of themselves. you can force measures onto a new page same as you'd force words onto a new page in a word processor - with a page break. But generally speaking, don't worry about the details of which measures are on which page until you're done with your score and can consider the big picture.