Only as many as are needed to physically rack the hardware and do hand-on maintenance. The people actually using the servers shouldn’t be located on-site.
A giant datacenter of AI scale will have a dozen or so contractors for physical plant on-site pretty much every single day as long as the thing is in operation. More if a refresh project is ongoing, which after a few years will be more less all the time.
They certainly are not high density employers, but these huge hyperscale facilities typically employ 150-300 people directly, and probably at least that many on average in contracting roles. They are massive facilities.
