At least, far fewer people than you might imagine. Despite billions of sign-ups, hardly anyone actually does anything on it - check this out:
There are about 2.2 billion Google+ profiles. Of these, about 9% have any publicly posted content. Of those, about 37% have comments on YouTube videos as their most recent activity, and another 8% have profile-photo changes. Only 6% of active profiles have any posted activity in 2015 (18 days so far). Only about half of those, 3% of active profiles, are not YouTube posts. In other words, 0.2-0.3% of all G+ profiles, about 4-6 million users, have made public posts in 2015.