Our first trip we used 9 rolls (36 print) but it has gone down to a more manageable level now, I usually get some developed whilst we're there to save on some of the bill when we get home.
If you're taking a camcorder take a spare battery and your charger to the main street camer store in Magic Kingdom and they'll charge it for you during the day, useful if you are going to watch the fireworks.
I dont' quite see why you want to buy 20 single use cameras at approx £7 a time why not just buy a cheap 35mm or APS camera, you can pick them up so cheaply (just looked in the Argos catalogue and the cheapest 35mm camera with flash is only £12.99) If it gets lost you haven't lost that much and you can always buy the disposables as a last resort.
As Fiona say though it is always worth taking a waterproof disposable if you are going to the water parks or on the rides.