Which numbering styles are there?

Plain (3), Page 3, Page 3 of 12, and roman numerals (iii) — stamped like a printed footer.

Can I number a merged document?

Yes — merge first, then add numbers here, and they'll run continuously across the combined file.

No — numbering happens in your browser.

Stamp clean page numbers on every PDF page — plain, Page X, X of Y or roman numerals — without uploading the file anywhere.