Zero: small scale reactor designs like pebble bed reactors are designed as "set and forget" - they're intrinsically safe, and only need to be accessed when they need refueling.

A model of deployment was that if you were setting up a town, you'd basically go dig a pit and lower one of these into it, pave over the top (or I suppose actually install some very heavy doors) and then build the town radially outwards from the reactor. The unit itself would be hermetically sealed - no employees, no systems to maintain. If it malfunctions or fails, then the idea is you drive out, pickup the container and drop a new one in.

Considering how many thousands of minimally-protected water treatment plants there are, and how much publicity a serious sabotage job would get, and how extremely few reports there are of sabotage actually happening...

Minimal. You're talking about highly trained, licensed nuclear power engineers.

That's a community in the US that can safely be estimated as 70% prior US Navy nuclear reactor techs and officers.