So much depends on a person's reasons for keeping bees ...
If you're 'just keeping bees' - then I'd say that 10 or so is a pretty good number - for exactly the reasons already stated: enough colonies to weather most calamities without needing to re-start from scratch, and yet a manageable number for treating/ winter feeding etc. without becoming too much of a burden.
But I find there's a problem with such a number if you plan on doing anything more than 'just keep bees'. If you want to raise queens, for example, the more colonies the better able to make an initial breeding selection. Then there's the issue of following the daughters (plural) and grand-daughters ('plural-plural') to see if your chosen traits have persisted.
One of my interests is comparing different types of hive, and again ten colonies really wouldn't be enough to do this (especially when queen-rearing as well) - and so the colony numbers just keep creeping upwards !
This is such a good thread topic, for 'hive-creep' has become an ongoing problem for me too. At the end of each season as feeding for winter begins (my only significant expense) I make plans to decimate the hive numbers down to maybe just 20-30 - but just as soon as the new season starts the familiar "oh - just one or two more extra nucs" begins, and so 'hive-creep' rears it's seductive head yet once again.
But - I've yet to find 'self-discipline' listed in any of the beekeeping supplies catalogues.

LJ