The most recent information available about this seems to be from 2010, and at that time the answer, amazingly, was just 35. You can see an interview where founder Jimmy Wales talks about how they manage to run such a large organization with so few staff, and what those 35 people actually do.

But of course this figure only refers to regular paid employees. Wikipedia does hire occasional workers, for example freelancers to write or improve some articles. Often it's just for one article at a time - you can get an idea from their jobs page of the kind of work they need.

Besides this, Wikipedia famously is mainly created by volunteers. The volunteer input does have to be monitored, of course - quite often you see a page that has been removed, or there is a note on it saying there is a problem with it and it may not be accurate. But most of the time the volunteer content is considered acceptable, though you aren't usually allowed to quote it in academic essays or official journals.