The Heizer Double Defense comes to mind. Two shot derringer and chambers .45 ACP, very devastating for it's size. Will knock a man down with one well place shot, twice in a row. Has a little compartment that hold two extra rounds stored on a rubber speed strip for rapid reloads. IMHO it's the best pocket gun currently made, about as thick as a tube of chap stick. I own and carry two, both the .45 versions though I think they make them in 9mm which is no slouch with good hollow points.
Oh, and if you are worried 2 isn't enough, don't. Studies have shown that just the presentation of a gun ends some like 95%+ of all potential violent encounters. If you do need to shoot, studies also show needing more than 2-3 rounds is rare. The .45 hits hard even with ball ammo, but doesn't kick terribly bad in the gun allowing good follow up shots. If you feel you must carry more than 2 rounds, do what I did and get a second one. They are so thin and small they can be concealed virtually anywhere.