Select the Master Canvas in the Canvasses drawer. Place a text box on the canvas wherever you want it to appear on any newly created canvasses. While the text box has focus, go to the
Edit>Insert Variable menu and select
Page Number. If you want to have X out of Y pages, following the code for the Page Number variable type the separator you want (e.g. a /) and then select
Edit>Insert Variable>Total Pages
The full code would then be <%#%>/<%TotalPages%>
If you only want it on a specific page, you can position a text box on just that canvas and insert the variable there.