How much alien blaster ammo is there in Fallout 3?
According to the Wiki article for the ammo: Alien Power Cell
(That link also contains the locations.)
In the core game:
362 power cells.
Entirely:
A maximum of 1152 alien power cells can be found in all expansions, without the raider glitch.
In Mothership Zeta, there are:
365 power cells.