Store your clay bar in the plastic container it came with, in a cool area, never anywhere that it can get too hot. This will keep it safe, clean and ready for the next time you need to use it.
Depending on the cleanliness of the surface you're cleaning, 1 bar should be good enough for safe use on 3 cars. If it looks dirty, it means it's time to replace the bar. Try not to overuse the clay bar.