Assuming your phone/camera is mounted in a stationary position, you could track all the moving objects fairly easily with background subtraction. I say "fairly" because you'd likely have to incorporate motion, color and texture detection to disambiguate between different people. A count of the moving blobs would give you a rough number of the people in a store. It would be an instantaneous count, so you'd probably have to take a moving average of the time series of the number of people detected to get a reasonably accurate value. Non-trivial, but doable.