I'd like to know the average number of smoke detectors per commercial
building.  Or, if you can find out the total number of smoke detectors
in use in commercial buildings, then I can divide by my estimate of
total commercial buildings (4.645 million, as reported by the DOE) to
get smoke detectors per building.

I've defined commercial buildings as all buildings except single and
multi-family dwellings (i.e., excluding houses, apartments, and
condominiums).