Type > Insert Special Character > Marker > Current Page Number. This would be built into a textframe on the master, and copied to the same position on the other masters, too.
Also, the textframe on the master page could have a numbered list defined as part of the paragraph style, and it would apply a number to an empty hard return in the textframe.
Also, if your numbers change and arent always consistent, you could make that same master page textframe contain a datamerge field that would match the correct number into the position on the page.