Last modified: 2009-07-12 12:10:45 UTC
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.
While of course this number should be available for every group, the number of sysops is already available through siprop=statistics.
Fixed in r53131.