how to convert {PAGE} to a page number on Microsoft 365 Word on Mac computer?

The following shows an intro with lower case Roman numerals. You can use any other format instead.

1. Select between the intro and the body of the document and go to Layout > Breaks > Next Page.
2. In the header for the body section, deselect Link to Previous. If Link to Previous is dimmed, check to make sure a section break was created.
3. 
   
4. Note: Headers and footers are linked separately, so if your page number is in the header, turn off linking for headers. If your page number is in the footer, turn off linking for footers.
5. Go to Header & Footer > Page Number, and then click Page Number.
6. Choose the position and alignment of page numbers. If you don't want a page number to appear on the first page, clear Show number on first page.
7. To choose a format or to control the starting number, go to Header & Footer > Page Number > Format Page Numbers.
8. To change the numbering style, select a different style in Number format.
9. To change the starting page number of the newly created section, select Start at, and then enter a number.
10. Select OK.
11. Select Close Header and Footer, or double-click outside of the header or footer to exit.

For more info on page numbers, see Page numbering in Word.

https://support.microsoft.com/en-au/office/add-different-page-numbers-or-number-formats-to-different-sections-bb4da2bd-1597-4b0c-9e91-620615ed8c05#ID0EDBD=macOS