Of course, it's impossible to know how many sensors the aerial mapping/intelligence folks are buying... Some of that work is commercial, for everything from real estate development to agriculture and forestry to ecological science (even those numbers are hard to get, but there's no reason they should be impossible. A largeproportion, however, is classified spy secrets.
Most developed countries are photographed regularly from the air as evidenced by google map pictures. In addition to what you see on google, agriculture uses pictures taken in several bands outside of the visible. The corresponding maps are translated to instructions for GPS-guided agricultural machines, e.g. varying the quantities of nutrients delivered to the plants within a single field. All these need quantities of pictures.
The cameras in the planes are arranged in pods of a few cameras, typically between 4 and 10. MF cameras are often used because the increased resolution, which directly translates into less flight time.
None of this is classified spy secrets. Covert operations do not use planes, which would need authorisation to enter airspace, but satellites. Satellites typically (but not always) use linear sensors, because the satellite regular movement makes it an ideal scanner head.
