LaTeX package to get the number of pages in a given PDF file

countpdfpages.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{countpdfpages}[2023/04/19 v0.1]
\RequirePackage{iftex}

\newcount\PDFPageCount
\newif\if@countpdfpages@pagenumread

\def\CountPDFPages#1{%
  \IfFileExists{#1}{%
    \ifluatex
      \PDFPageCount=\directlua{tex.print(pdfe.getnofpages(pdfe.open('#1')))}\relax
    \else\ifxetex
      \PDFPageCount=\XeTeXpdfpagecount"#1"\relax
    \else\ifpdftex
      \pdfximage{#1}%
      \PDFPageCount=\pdflastximagepages
    \else
      \immediate\openin\@inputcheck="|extractbb -O #1"\relax
      \@countpdfpages@pagenumreadtrue
      \loop
        \catcode`\%=11\relax
        \read\@inputcheck to\@tempa
        \catcode`\%=14\relax
        \ifeof\@inputcheck
          \@pagenumreadfalse
        \else
          \expandafter\@CountPDFPages\expandafter{\@tempa}%
        \fi
      \if@countpdfpages@pagenumread\repeat
      \immediate\closein\@inputcheck
    \fi\fi\fi
  }{%
    \@latex@error{NOT FOUND: #1}%
  }%
}

\catcode`\%=11
\def\@CountPDFPages#1{\@@CountPDFPages #1%%Pages:\relax}
\def\@@CountPDFPages #1%%Pages:#2\relax{\def\@tempb{#1}\relax
  \ifx\@tempb\@empty
    \@@@CountPDFPages#2\relax
    \@countpdfpages@pagenumreadfalse
  \fi
}
\def\@@@CountPDFPages#1 %%Pages:\relax{\PDFPageCount=#1\relax}
\catcode`\%=14