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.
Not for all. Veneers, and Lumineers is just one variety, are for cosmetic correction, brightening. They do have a finite life span and at some point will require replacement. Have you considered seeing an Orthodontist to have your teeth straightened, and then having them bleached? May be less expensive that veneers.
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.
No. Only for visible teeth when you smile. Usually between 6 to 10 teeth in one arch.