3 legged chairs don't rock but the legs need to stick out much further to give stability against tipping over. 4 legs don't need to stick out as far and 5 legs can stick out less again, for the same stability. It's all about the size of the footprint and the amount of material used in the construction. After that the number of legs starts to add to the amount of material used and is less cost effective. Having more legs does not mean that each leg can be less solid as any 2 legs need to be able to support the weight at the point of tipping over. Of course, once you get more than 3 legs you need to have a flat floor to stand it on to prevent rocking.