How many Stardust are there, and what do I get for collecting them all?

There are 50 stardust in the game. For every 10 that you collect, you can visit Martel to gain a new accessory.

• 10 stardust - Physical Ring (Raises maximum HP 50%)
• 20 stardust - Amulet (Doubles maximum MP)
• 30 stardust - Wargod Sash (Raises SP by 50%)
• 40 stardust - Rainbow Earring (Prevents all abnormal status)
• 50 stardust - Vanishing Stone (Gives access to Magician Faust, an optional boss at the Tower of Flanvel in Kashua Glacier.)

This article in the Legend of Dragoon wiki provides more information on Stardust, as well as a guide on where to find all 50.

http://legendofdragoon.wikia.com/wiki/Stardust