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.