If I'm carrying my M&P 40, I'll always carry an extra magazine in my packet in a desantis pocket-magazine holder. That give me 31 rounds of .40.
If clothing permts, I will also carry a backup gun, an M&P 340 j-frame, with 2 5-round speed strips for reloads, for a total of 15 rounds of .357 magnum. So, most of the time I have 46 rounds on me. Some people find that excessive, but I like the peace of mind. Occasionally I will just carry the 340 and a few speed strips witout the semi-auto.