\version "2.18.2"
% produce a pagenumber with full pagenumber: n / m
% rough idea sketch by Jan-Peter Voigt


% record the highest pagenumber
#(define-markup-command (record-page-num layout props sym)(symbol?)
   (let ((pn (ly:chain-assoc-get 'page:page-number props 0)) ; current page number
          (smn (if (defined? sym) (eval sym (current-module)) 0))) ; current stored value
     ; max pagenumber | stored value
     (if (and (integer? pn)(> pn smn)) (set! smn pn))
     ; set variable
     (module-define! (current-module) sym smn)
     ; report pagenumber for debugging
     (ly:message "page ~A" smn)
     )
   ; this an empty markup
   empty-stencil)

% get the stored value delayed
#(define-markup-command (get-delayed layout props sym)(symbol?)
   ; provide space for the delayed value like in table of contents
   (let* ((gauge-stencil (interpret-markup layout props "123"))
          (x-ext (ly:stencil-extent gauge-stencil X))
          (y-ext (ly:stencil-extent gauge-stencil Y)))

     ; produce delayed stencil
     (ly:make-stencil
      `(delay-stencil-evaluation
        ,(delay (ly:stencil-expr ; this will be run, when all pages are calculated
                 (interpret-markup layout props (format "~A" (eval sym (current-module))))
                 )))
      x-ext y-ext)
     ))

\paper {
  oddFooterMarkup = \markup {
    \record-page-num #'pn \fromproperty #'page:page-number-string
    /
    \get-delayed #'pn % This will be one page high!
  }
}

\relative c'' \repeat unfold 4 { \repeat unfold 240 c4 \pageBreak }