They may opt for one camera when they shoot inside of Silvercup Studios since it's easier to control an interior set, but that day when it was outside in NYC streets, there were two. For Ugly Betty they were also shooting the exterior scenes with multiple cameras. It would be insane to try to get everything done in an unpredictable environment such as NYC and with such tight deadlines with just one camera, but I'm sure there are some shows that do that on a daily basis.