Driving many LEDs from a battery can be a pain to do, especially if you need many of them to have the same brightness. One way to do this is to use current mirrors for each individual LED, and drive them from the battery voltage only. Another is to drive them with a dc-dc boost converter in a current feedback loop.
This app note from Maxim starts by explaining LED driving basics, and provides examples for various different topologies. From driving 3 LEDs with a current mirror, to driving and controlling 6 sets of LEDs trough a highly integrated driver with analog and digital dimming controls.