add page

Add/Measures: https://musescore.org/en/handbook/3/measure-operations
Alt+Shift+B
See also: https://musescore.org/en/handbook/3/breaks-and-spacers

To be clear: just as in a word processor, you don't normally add pages manually, they get added automatically as needed. So once you composition is long enough to spill over onto another page, another page will appear. At most, if you didn't specify enough measures when you created your score, you will need to add measures manually.