E-6 Mercury in the air!

"The E-6B is a dual-mission aircraft capable of fulfilling either the E-6A mission or the airborne strategic command post mission and is equipped with an airborne launch control system (ALCS). The ALCS is capable of launching U.S. land based intercontinental ballistic missiles."

[link to www.military.com (secure)]

there are a minimum of four in the air at any given time.

They're always up, operation looking-glass
I've seen as many as 6 at a time on ADSBexhange