How to add page number for a pdf file whose page sizes are different

Here is an incgraph solution:

• in the first example, the page number is put inside the picture,
• in the second example, the page number is put outside the picture, hence enlarged the page size.

documentclass{article}
usepackage{incgraph}
usepackage{tikz}

igrset{
  my page number/.style={
    overlay={
      node[anchor=south] at ([yshift=5pt]page.south) {thepage};
    }
  }
}

begin{document}
content
incgraph[my page number][page=1]{example-image-a}

igrpage{%
  begin{tikzpicture}
    node[inner sep=0pt] (pic) {includegraphics{example-image-b}};
    node at ([yshift=-10pt]pic.south) {thepage};
  end{tikzpicture}%
}
end{document}

After a lot of struggle i could do it by the following.

The answer is based on Change Page Size Mid-Document: Fancy Header and Footer Horizontal Width

But still i have to struggle a lot and finally using left=0mm,right=0mm,top=0mm, bottom=0mm,footskip=1mm in newgeometry did the job

Also showframe helper me to visualize the textarea.

The problem is fancyhdr does not adopt to the mid document page size changes. and then understading the geometry package was the key

But still i am half knowledge. and left few questions i have as comments in the script. I dont know what is their purpose. but trail and error i understood they are required.

documentclass{scrartcl} % THIS IS REQUIRED
% QUESTION:
% documentclass{article} pushes the pagenumber down
% I dont know the reason
usepackage{geometry}
usepackage{pdfpages}
usepackage[automark,headsepline,footsepline]{scrlayer-scrpage} % THIS IS REQUIRED
% QUESTION:
% without the above line the page number is pushed down /out of the page
% I dont know the reason
% usepackage{showframe}

begin{document}

newgeometry{layoutwidth  = 432pt,
layoutheight = 179.64pt,
left=0mm,right=0mm,top=0mm, bottom=0mm,footskip=1mm}
includepdfmerge[fitpaper,pagecommand={thispagestyle{plain}}]{/home/simha/latex/test.pdf, 1743}

newgeometry{layoutwidth  = 432pt,
layoutheight = 71.63999999999999pt,
left=0mm,right=0mm,top=0mm, bottom=0mm,footskip=1mm}
includepdfmerge[fitpaper,pagecommand={thispagestyle{plain}}]{/home/simha/latex/test.pdf, 1744}

newgeometry{layoutwidth  = 432pt,
layoutheight = 192.12pt,
left=0mm,right=0mm,top=0mm, bottom=0mm,footskip=1mm}
includepdfmerge[fitpaper,pagecommand={thispagestyle{plain}}]{/home/simha/latex/test.pdf, 1745}
end{document}

The output is

Also for how to add space at bottom (eg 15mm for page number) in a pdf with different page sizes and then add pagenumber, refer answer https://tex.stackexchange.com/a/556945/221200

My basic idea would be to wrap a tikzpicture in the pagecommand of the includepdfmerge. In addition you would need the remember picture, overlay argument to write on the pdf and the pagenodes function of tikz to get the position at the bottom of your current page. This mechanism should be able to handle different page sizes of your included pdfs and put the pagenumber always at the bottom (set the yshift argument to a value that suits you).

MWE based on your code

documentclass{article}
usepackage{pdfpages, tikz}

begin{document}
includepdfmerge[fitpaper,pagecommand={
   begin{tikzpicture}[remember picture, overlay]
     node[yshift=15pt] at (current page.south) {thepage};
   end{tikzpicture}}]{/var/tmp/pdfjam-7FXkbO/source-1.pdf,-}
end{document}