The implication is that, considering three equally manned shifts, more
than 5,000 controllers are watching the skies over the United States for
us at any given time. Impressive.

But, of course, there aren't three equally manned shifts. Many towers and
TRACONs are closed at night, the airports become uncontrolled and the
airspace is assumed by the overlying ARTCC. ARTCCs combine sectors so
that one or two controllers handle the functions of a dozen or so during
peak hours.

I get your point.
Can you give me an estimate, like 50-percent on days, 30 on swing and 20
on graveyard? [Hmmm, that may be a common name for the wee-hours shift but
it seems inappropriate here, somehow.] That is, without focusing on any one
part of the country. My (usually perverse) logic tells me the NE is more
heavily trafficed than the SW, and East Coast more than Left Coast.
That's probably to be found in the document Mr. Natalie referenced
earlier, but this might be an easier way to get a lead the information.