How many chains before the cassette is replaced?
This is really going to be different for each person. If you replace your chain early enough, there should be very little wear on your cassette and you can go more chains between cassettes. If you let the chain go for a long time, you pretty much have to replace the cassette with the chain.  Each one should be replaced as they are needed depending on wear.  The amount of wear on the cassette is going to depend a lot on your riding style. If you spend a lot of time in one gear, that sprocket is going to wear out. If you use all the gears evenly, the cassette as a whole will last more miles.