1. Open the report in Designer.  
2. At the top, click the Insert tab.
3. Drag the Page Footer icon and drop it anywhere on the page. It automatically appears at the bottom of the page, gray, named PageFooterBand1.
4. From the toolbar, drag the Text icon down and drop it inside the PageFooterBand1 section.
5. The Text dialog appears.
6. If needed, click to select Expression.
7. In the blank text area to the left, type (or copy/paste) one of the lines below that has {braces around it}

option a gives the current page number:
{PageNumberThrough}

option b gives "Page (current number) of (total number)" e.g. Page 1 of 3
{PageNofM}

8. Click OK.
9. File menu, Save.
10. Run the report. You should see the page number in the bottom of the page.

Additional Information for advanced users:

This webpage has other available system variables. Stimulsoft makes the report module used in Wasp's Cloud system.
https://www.stimulsoft.com/en/documentation/online/user-manual/index.html?data_data_dictionary_variables_system_variables.htm