I went to one of the BCC Moving On courses recently, where we had a brilliant talk from the nurse who runs the local lymphoedema clinic (there are causes besides BC, of course). She explained that there are other lymph nodes in your arms and it's usually just the cluster under your arm that is removed. The remaining nodes have to work harder, which is why it's important not to over-work them. This is very simplified, by the way, and I would really recommend the Moving On courses for a better understanding and for advice on avoiding lymphoedema.
Of course some people have to have additional nodes removed, which increases the overall risk.
Not sure where else to post this but if the majority of people are supposed to NOT get Lymphoedema how do their bodies drain the fluid away etc? I am really puzzled by this - do they grow new pathways or what?