Can it be necessary to have several cameras?
Several bikes or many books one can understand
Yep. I've lost count, but I know people with bikes who probably have lost count of their bikes as well, certainly if you count in major spare interchangable components like wheels, like you do with cameras with interchangeable lenses.
If I try to count up, I think I have six bodies- three film SLRs I don't use but can't bring myself to part with as one day I 'might use them again' - (I know people with bikes like that), and three DSLR bodies. Four lenses for the oldest film slr. and about seven working lenses that fit all the other five, film or D, and a broken lens I keep for spares. Plus a compact when I want something less weighty.
Leaving aside the film stuff, which is sheer nostalgia, I needed 2 bodies for work in case of a failure on a critical job, so I had 2 identical ones. The very new one is an upgrade to those, and at least one of the old two will be kept as backups for that one. I really only use about five of the seven lenses, and only three on a regular basis, but they really do all do different jobs. I really ought to get rid of the other two, as I'd never use them now. They were cheaper versions of the ones I use now which I bought while I saved up for better lenses. But I haven't got round to it yet...........however, we may be moving house, so that might just get me doing a clearout.
oops, just remembered another lens.