Currently, there is not a page number feature included on the screenwriting page.
It has been mentioned in the past and is being considered for a future update.
In the meantime, you can open the print dialogue box to view the page numbers.
In the upper right hand corner of the page you will see three dots, the More button, click this.
In the dropdown menu click the Print button. This will open the print dialogue box that will have the script broken into pages.