Yes we know that TCPDF can get the number of pages :), but it's different because a PDF is created using concrete sizes, unlike DOCX that sizes are calculated when the document is opened.
Maybe if you use a macro, you have an Office Server and you use COM functions you can get the number of pages. I think it's a little difficult and wont work in other OS than Windows but if you try it and works please let us know.
Regards.