I've never really thought about it. I would have to say five or six, as I don't like derringers, a wheelgun will hold at least five, and an auto will hold at least six. I prefer to carry at least one reload, but have ran out for a minute without one.
Since I don't like derringers, I think I'd be hard pressed to find something holds less than five or six rounds. The two smallest pistols I've owned were a Taurus Model 85 and a Kahr PM9. The PM9 is long gone, later replaced with a P9.
If I cant conceal a Model 85 or a P9 in what I'm wearing, I'm not dressed appropriately to be in public, so you wont find me in public, and a smaller pistol is useless to me.
I'll take a maglight over a single shot derringer. If there's a good chance they're close enough to hit with the derringer, I take a step can reach them with the club, er, maglight.