Most people don't bother using a torque wrench to tighten a thermostat housing.
It is more important to be careful and tighten each bolt a little at a time. You do not want to warp, crack, miss align the part you are bolting.
It does not take a lot of torque to seal a thermostat housing. I would guess somewhere between 15 and 20 ft lbs.
It is often hard to find such torque specs for specific vehicles.
For bolts that you cannot find a specified torque spec, you can refer to a standard bolt torque chart. The charts are based on the diameter of the bolt threads not the wrench size used to fit the bolt head. The charts also depend on the hardness of the bolt material. Also keep in mind the material you are bolting into.
Here is one such chart...
Keep in mind that thermostat housing bolts are just sealing a gasket. They are not holding the entire car together.
After turning wrenches for many years, most mechanics just know by feel.
There are cases where a torque wrench is mandatory. Like head bolts, wheel bearing/hub nuts, rod caps, main bearing, etc.