Do you have to get lumineers for you entire mouth?

No... Lumineers are generally done for the front teeth and premolars. There are generally two premolars located behind each canine or eye tooth. I do not trust them on the molars where the forces of biting are much stronger.
Depends. Lumineers are a cosmetic procedure only. How many you have done is a cosmetic issue. If teeth are damaged, you may need other types of restorations.
No. It depends on the level of brightness you desire, the number of teeth that show when you smile, and how dark your teeth are without them.