I try to use all the cameras I own, which is a lot more than 20. My intention is to rotate through them, but a majority of cameras have so far only seen one roll of film through them -- sort of like my CD collection. The ones that I have been going back to time after time these days are:
Leica M6 TTL
Pentax LX
Pentax MZ-S
Samsung GX-1s DSLR
Minolta CLE
Contax RTS II
Hasselblad XPan II
Mamiya 7II
They are being used more to a large extent because they are the ones that I know are fully reliable and I have relatively larger lens set to go with them. A lot of the exchangeable lens cameras I own I have a limited number of lenses available for them. And of course, a lot more cameras I own I don't trust them too well for important occasions as they are probably in need of a CLA. Here are some that I have used in the past year:
Olympus 35 RD, 35 RC
Konica Autoreflex T3, Autoreflex T4, Auto S3
Minolta XE-7, Hi-Matic 7sII
Kodak Retina IIIc
Voigtlaender Vitomatic IIa
Nikon FM3A
Contax RTS III, AX, N1
Aka Akarex III
Wirgin Edinex III
Rolleiflex SL35, SL3003
Topcon RE-Super
Yashica Electro 35GX
Praktica VLC3
Zeiss Ikon ZI
Cosina Voigtlaender Bessa R3M, R4A
Bronica RF645
Mamiya Press Universal
My days might go by more happily had I not owned this many cameras and having to worry about keeping all of them in working condition🙂
-- Yefei