I've always managed with just the one (bike, not child, I hasten to add) which is strong enough to carry camping kit.
Oh, and there's one that is too light to carry camping kit, and hence slightly faster and has aerobars for long distance.
That's all, really.
Oh, and there is also the one in the middle, which can carry a bit of luggage and is reliable, that's mostly used on Fridays rides.
So that's the lot.
Well also the one for going round town, which has many locks, one of which was a present on a Fridays Tour, that's used every day.
That's all a rider needs, to be honest.
[EDIT] Just remembered another one. Shan't bore you any more. [/EDIT]