Stephen Reid recently commented that the number of flashpoints (4 man instances) in the game at launch will be in double digits. This could mean 10, or it could mean more. These start at lvl 10 with the Black Talon (I know this is a Sith FP, not sure if its available to Republic either) and progress through to 50.
Also, all Flashpoints have a 'heroic' setting as I understand it, meaning we can go back and do the lvl 10 FP's at 50.