[Python] Adding page number while merge a pdf with PyPDF2

I am merging units of information(.PDF) into a sigle PDF. I have a lot of variables whithin a lot of filters (you can select just one of each filter) that need to be used in order to define the complete manual, look : Manual Generator in portuguese

My major problem is basically add a page number to the PDF while its merging itself, i was using the code here and here, but unsuccessfully The simplified code i am using is:

from PyPDF2 import PdfFileWriter, PdfFileReader
import io
from reportlab.pdfgen import canvas
from reportlab.lib.pagesizes import A4

print(var1.get(), var2.get(), var3.get(), var4.get(), var5.get(), var6.get())
merger.append(Modelo) #filter one
merger.append(Escada) #filter two
output = open('ModeloMerg.pdf', 'wb')
merger.write(output)

pdf = PdfFileReader('ModeloMerg.pdf')

packet = io.BytesIO()
can = canvas.Canvas(packet, pagesize=A4)
can.drawString(10, 100, "Page" + str(15)) #just a random test number
can.save()
packet.seek(0)

watermark = PdfFileReader(packet)
watermark_page = watermark.getPage(0)



for page in range(pdf.getNumPages()):

pdf_page = pdf.getPage(page)
pdf_page.mergePage(watermark_page)
pdf_writer.addPage(pdf_page)

with open('out.pdf', 'wb') as fh:
pdf_writer.write(fh)

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