Does anyone KNOW how many amps a 120 amp rated electric furnace draws? Do they continually draw 120 amps if all elements are lit? Etc. Curious. Yes, I could post this on some electircal forum but have not found one.
I would search the makers website for the installation and operation documents, they will tell you what you are looking for
It should be on the nameplate (Watts)
Whether you selectively can turn on part of the elements depends on the thermostat as much as the toaster heater itself. The elements are going to be subdivided into 48 amp loads each or less. You probably have three 40a banks (9600 watts). You may find this is not really 120a tho when you read the name plate. You may find this is really three 8kw elements and only draws 100a or some other combination. The 48a max for each element is the ruling code requirement there .
Again it should all be on the label.