The number the developer gave in a press release was "20-30." I find that reasonable as a very large Facebook data center near me has a permanent staff of around 50. Keep in mind that these large DCs use contractors for the majority of the work, which unfortunately doesn't really help with employment because the contractors mostly come in from out of state (there is a HUGE temp labor market for traveling IT technicians and skilled crafts get hired mostly from big national outfits that just send whatever crew is available next). It is good for the hotel business though.

Once it's built, it basically runs itself.

You have a guard, some remote hands, maintenance, maybe additional security or two, times 4 for the various shifts. 30 sounds about right.

Even 20 years ago the datacenters I worked with often had fewer employees onsite than "visitors" - because they rented out racks.

From the Maine Monitor:

[…]the data center would have employed only about 30 workers, the city estimated.