Reported via a comment at https://moodle.org/plugins/enrol_groupsync

It seems to me that there is a small bug. When an enrolment method is created for a given course, the number of users shown is always zero, although the number of users in the corresponding group is not zero.

I found this counter-intuitive as well, although I can understand the reasoning and maybe there are technical limitations. Would be good to explore possibilities to amend the number displayed there, if there are any. If nothing else, than displaying nothing would be better than this.