The shotgun seen in this movie is a sawn off Browning 2000. O'Niel (Sean Connery), Sheppard (Peter Boyle), and the other police all use them. The two hitmen hired to kill O'Niel use them as well, with infrared scopes attached. It seems likely that there was only one used in the entire film, as all the shotguns used look exactly the same. In the scenes where multiple guns are present it's likely the others are props. Interesting to note is that one of the hitmen is P.H. Moriarty, who played Hatchet Harry in Lock, Stock and Two Smoking Barrels.
O'Niels shotgun is tapped for the giant scope mount that the hitmen use. You can see the small screw hole just forward of the loading port. This supports the "one gun" theory.
