I think it's six at the moment. Nothing very exotic.
Polar CS100 cadence transmitter - limited range
My Polar Cadence sensor won't transmit beyone about 18 inches. In other words, if I move the receiver near it, it works but not when it is on the handlebars.
Does this mean that it is basically 'worn out' and needs replacing? I presume I can't change the battery etc.?
People have cut the Polar transmitters open and replaced the batteries. A web search should find pictures. IIRC Polar also has some sort of exchange program which is less expensive than new sensors via retail channels.