You can run as many as you want, it depends what you put on those outlets
for instance, 10 outlets with 10 20w pumps = ok! 10 outlets with 10 250w heaters = bad
Add up your total wattage divide by 120 to get the amps... leave a bit of wiggle room for spikes, i.e. when lights turn on they pull more than their nominal operation current.
Remember though if you only use one GFI, if it trips for any reason you loose power to all of the downstream outlets.
I always suggest multiple gfi's on a tank setup. That way if something does cause the gfi to trip (and you can get false trips with some lighting systems), you can still have power to other parts of the tank.
