That said, it will be very easy to add a master page: most will be using a Text Frame. Let's see how to use it in detail:
Go to your master page.
Add a Text Frame where you want to add the page number.
Double-click within the frame and activate the Content mode.
Go to Insert | Character | Page Number and you'll see a # in the frame.
That's good, set it nicely: you can add other text in the frame such as "page" to get something like "page #", or place shapes around or under the number.
Exit the Master Page mode and you'll see that the page number will be applied to all of the pages on which your master page is applied. The # has been replaced by the actual page number.
By adding a Text Frame, we just placed the number. We could have inserted the frame anywhere in the page, even in non-practical places. It can be the size we want too, put to the background, or have the main text of the page flowing around. If the Scribus way needs more steps than...