Protocol extensions for a mobile Internet have been developed within the IETF, but a standard design of mobile multicast is still awaited. Multicast routing, when adapting its distribution trees to moving listeners or senders, needs to newly established forwarding states. In this paper we quantize the number of states minimally required for servicing listener or sender mobility. Independent of the actual routing protocol in use, these results serve as an inherent measure of complexity for multicast mobility management. Results are based on current Internet measurements and a topological analysis from network simulations. They show a surprisingly low mobility overhead as compared to general multicast forwarding state management.