It depends on the following:
1. Current draw of the machines,
2. Your total circuit load (how many other things are hooked up to the outlet and any other outlet that shares the same breaker)
3. Cricuit load limit based on the breaker on that circuit.
Most household 3 prong plugs are usually connected to a 15 amp circuit. With 120 volt circuits, that is a maximum draw of 1800 watts.
A while back I posted a thread on average current draw on various machine types. Assuming you have nothing else connected to this circuit and an average load of 300 watts per machine, then you are looking at a maximum of 6 machines. Remember just because there's nothing else plugged into the power strip, does not mean that something else is no plugged into another outlet that shares the same breaker. You need to know what else is using that particular circuit in order to be totally sure of your loads.
Here's the thread with average loads on different types of machines.
http://newlifegames.com/nlg/index.php?topic=4499.0