I have eight (all different sizes and shapes). My friend has one. She thinks I have an excessive amount. I think she must have to constantly wash her one bowl up (and it must get lonely in the cupboard).
I use mine loads! I currently have one small and one medium one in the fridge (holding tinned pineapple rings and mashed potato), one big one with a flatter bottom on the side acting as an impromptu cold-water steriliser with water bottle tops in, two medium ones have been washed up from dinner (making gravy and custard respectively), and I've just weighed out flour into my biggest one ready to make scones tomorrow.
This is obviously light-hearted, but if you lot all have multiple bowls I think I know what I'm getting her for her next birthday. 
And if you only have one and really and truly are happy with it and never feel the desire for another one, what do you use instead?