Two or three. Ideally, I have one in the camera, one in the charger, and one in the bag. It really is more than I need; two would do. But I use the battery until it blinks red, telling me I need to change it, so I don't always leave the house with a freshly charged battery; thus I need one fully charged in the bag, or in a pocket. But I keep extra batteries in my bag for film cameras too, except the fully mechanical ones that I use with a hand-held meter. Lugging around a camera with an electronic shutter that I can't use is too frustrating not to carry batteries.