Pages are not numbered in the "screenplay" format.

The "screenplay" format outputs plain text, so numbering is
problematic; there are no pages yet.

Perhaps a text-to-pdf filter (or using an existing filter from
elsewhere) would be a good first step. The ability to number could be
added at that stage or as a post-process filter.

It might be necessary to annotate the plain text form to indicate
where page breaks are allowed; dialogue and actions should not
ordinarily be split in the middle.