1. number of subscibers in the network does not depend on the amount of channels.
2. same as 1. You can cover 7 billions subscribers with 20 channels. Even more.
3. frequency reuse means that you can reuse one frequency, or a set of frequencies, if there is enough attenuation between the place where you are primarily using them and the place were you want to reuse them. It is the ONE fundamental concept of GSM. If you are not familiar with that then you’re certainly better off with learning the basics through internet or with a book, rather than asking here 🙂
good luck !