Answer:
In theory, as many as you want. However, the circuit supplying the receptacle(s) you are using has a maximum capacity before the circuit protective device opens (usually a fuse or circuit breaker). Since appliances come in many sizes / wattages, there is no single answer.
Some large appliances require a dedicated circuit with only one single receptacle, so that other things can not be plugged in at the same time on that circuit.
Conversely, a 15 amp general purpose circuit could probably handle 100 clocks.
first determine how many amps the circuit is fused at. then how many watts the appliances use that are on the same circuit. then you take the watts divided by volts (120) = amps....add them up.