There's currently no way (that I can see) to return the number of people in a group (like sysop) via the API. The wiki has {{NUMBERINGROUP:sysop}}, which has led some to suggest hacks like http://en.wikipedia.org/w/api.php?action=parse&text={{NUMBERINGROUP:sysop}} to pull the data from the API.
meta=siteinfo seems like an appropriate place for something like sinumberingroup=sysop or something.
Version: unspecified
Severity: enhancement