4 shotgun shells

The basic unit of ammo for both the shotgun and the super shotgun is 4 shotgun shells. At the I'm Too Young To Die and Nightmare! skill levels, the item gives 8 shells; however the pickup message still says "4".

For gameplay considerations, the two shotguns use shells to unequal effect, and the shotgun operates differently for the player and the shotgun guy. When wielded by the monster, the shotgun produces only three pellets when fired, while in the hands of the player the weapon fires seven pellets per shell. Furthermore, the super shotgun fires a hefty twenty pellets, potentially causing nearly three times the damage the shotgun inflicts, while using only twice the ammunition.

Each pellet does between 5 and 15 hit points of damage (determined using Doom's pseudorandom number generator); the same as a bullet fired by the pistol or chaingun.

Data
In classic Doom, sets of 4 shotgun shells are first encountered on these levels:

The IWADs contain the following numbers of the pickups:

Doom RPG
The shotgun shell pickup appears in Doom RPG. It gives the player either 4 or 10 shells.