At last count, I think I had about 25. No, I do not use all of them (some take obsolete film formats that are no longer available, e.g., 126 film), and save for the Leicas, the DSLRs and the remnants of my m4/3 kit, none are worth much at all.
You could say I have three systems, and a bunch of assorted old cameras. The three systems (in order of how much use they get) are:
Leica M - M6TTL, M9P, 24mm and 50mm Elmars, 35mm and 90mm Summarits, and a Zeiss 50mm 1.5 C-Sonnar.
Canon EOS - 50D, T1i (500D), Elan IIe, 17-40 4.0 L, 70-200 4.0 IS L, 10-22 3.5-4.5 EF-S, 28 1.8, 60 2.8 EF-S, 24-85 3.5-4.5.
Canon FD - A-1, AT-1, AE-1P, TX and a whole bunch of FD lenses.
There´s also the remnants of my m4/3 system: Lumix GF-3, 20mm Lumix, Oly E-P1 and kit zoom.
The rest is a ragtag assortment of old rangefinders, SLRs and box cameras that have been inherited, received as gifts, or bought very cheap.
The Leicas see the most use. The Canon DSLRs are only used when taking photos for the Museum or if flash is necessary. The Canon FD system sees very occasional use, and has seen no use since I bought the M6. Everything else sees no use at all.
If you are curious, this is what they look like:
Camera Collection - a set on Flickr
Cheers,
Antonio