it depends on the bike's use, for a local runabout one is enough. for more "serious" rides and hilly terrain having two brakes is much safer.
^ Agree completely.
My 1 brake fixed (an Orbit track bike with a new fork) isn't used for anything super-steep, and with no redundancy I pay a lot of attention to brake maintenance.
The way the single brake performs is also important; it's very helpful to have a powerful, but progressive, brake that rarely causes surprises. A grippy front tyre helps too. The Orbit is a delight to ride around town, or anywhere else apart from steep descents, and I don't feel any lack of stopping power.
My Genesis fixed has two brakes (necessary as it also gets used on freewheel). The single brake on the Orbit is much nicer to use than the vee brakes on the Genesis which while powerful enough somehow lack the feel of the Shimano brake on the Orbit.