If you open the Pages panel, (Window>Pages) you will see thumbnails of your master pages and your document pages. In new documents, there is usually a master called A-Master and one called None. If you drag the thumbnail of the None master onto the thumbnail of a document page, it will apply that master to that page. Presumably, you have made your page numbers on A-Master, so this will usually do the trick. If you have elements on A-Master that you need on the first page, but just not the page number, you can duplicate A-Master (which, by default will give you B-Master, but you can change the names if you like) and remove the page number on the duplicate and apply that to the document page.
It might also be worth noting that you can apply master pages in the fly-out of the Pages panel. Go to the fly-out triangle and go down to Apply Master to Pages…, then select a master to apply and the range of pages (1-8 will give you all 8, 1, 3, 5, 7 will be the odds, 1-3, 4, 6-8 if you want continuous ranges and individual pages).
You can manually override the frame by holding CTRL/CMD SHIFT and clicking on the frame, then delete it.
Especially if it concerns only a page!