https://tex.stackexchange.com/a/483189/35864

overallcitetest.tex

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[style=authoryear-comp]{biblatex}

\addbibresource{biblatex-examples.bib}

\renewbibmacro*{cite}{%
  <\the\value{overallcitecount}/\the\value{overallcitetotal}>
  [\the\value{citecount}/\the\value{citetotal}]}

\begin{document}
% leaking counts with \cite (probably the smuggling?)

A \cite{sigfridsson,worman,nussbaum}

B \cite{sigfridsson}

C \cites{sigfridsson,sigfridsson}

D \cites{sigfridsson}{worman}

E \cites{sigfridsson,sigfridsson}

RESET \setcounter{overallcitecount}{0}\setcounter{overallcitetotal}{0}
% comparison with \cites

A \cites{sigfridsson}{worman}{geer}

B \cite{sigfridsson}

C \cite{sigfridsson,worman}

D \cite{sigfridsson}

RESET \setcounter{overallcitecount}{0}\setcounter{overallcitetotal}{0}
% \forcsvlist over the list of keys does not drop duplicates, hence
% may give inflated values

A \cites{sigfridsson,worman,sigfridsson}
\end{document}

There is precedence for using global variables and (re-)initialising them at the beginning of the relevant commands (at least on a .cbx level, see for example https://github.com/plk/biblatex/blob/master/tex/latex/biblatex/cbx/authoryear-icomp.cbx), so that would not be unnatural.

Unfortunately, we only have a limited test suite written in Perl. https://github.com/plk/biblatex/tree/dev/obuild. I usually run it as follows from the root of the git folder

./obuild/build.sh install 3.13 ~/texmf
./obuild/build.sh test
./obuild/build.sh testoutput

You'll need a matching version of Biber in your PATH. If you start from current dev that would be the dev version from sourceforge (https://sourceforge.net/projects/biblatex-biber/files/biblatex-biber/development/binaries/). If you start from master Biber 2.12 should be fine.

The test suite only runs on the files in https://github.com/plk/biblatex/tree/dev/doc/latex/biblatex/examples and thus only tests a very limited subset of biblatex's functionality. Joseph worked on l3build tests a while ago (https://github.com/plk/biblatex/tree/l3build-tests), but since most things we'd want to test are typesetting-related and not of the pure function evaluation type that proved difficult.

Sorting multicites is a feature request that comes up from time to time. People see that \cite{sigfrisson,worman,nussbaum,geer} gets sorted if the turn on sortcites=true and then also want \cites{sigfrisson}{worman}{nussbaum}{geer} to give the same result (plk/biblatex#214 is a bit low on details, but https://tex.stackexchange.com/q/65809/35864 has an MWE that shows what I mean). I thought that if we are digging in the implementation of the multicite commands anyway and have to look at sorting we might as well look if something like this is feasible. But there are quite a few conceptual issues here.

Okay, I'll make the counter global with initialisation at the beginning of the citation command.

The test suite is something to start with, I'll use it. Random thought: testing typesetting is tricky, but I think that the output of \loggingoutput might be something to start with. Given the same settings the boxes TeX make should remain the same. Plus, with pdfTeX and LuaTeX it is possible to have reproducible PDFs (https://tex.stackexchange.com/q/440270/134574), so it might be used as well. I'll learn how to use l3build to do something about that.

I think that sorting the multicites will be awkward, but it can be done. However I believe that it should not be activated by sortcites. I think that a new option (a good name would be IKnowThatThisIsn'tThePurposeOfMulticitesButI'llDoThisAnyway ;-) should be added to make that work. Certainly possible, given an enough amount of code :)

I'm working on it. Right now I'm trying to understand how the sorting algorithm works to start messing around with it. As soon as I have something new I send you.