How many joints are in a crayfish's leg?

No - or, not "only" six. Lobsters belong to the order Decapoda (literally, 'ten feet'), an order of crustaceans within the class Malacostraca. Remember that the "legs" may have significant specialization and not necessarily resemble a process that the organism uses for walking - such as swimmeretes, or front claws; in this case it might seem the lobster has four pairs of legs for walking (pereiopods) and one pair for grasping and manipulating (chelipeds).