Arduino Duemilanove & PowerSwitchTail(adafruit)

I have had my Arduino Duemilanove($45) for a month. I just bougth a PowerSwitchTail from Adafruit($20). I hooked the PowerSwitchTail between my Aduino and a 110 volt lamp and now my system is working. With the PowerSwitchTail, my Arduino turns the lamp on and off in acccordance with the program or sketch I wrote.

Now with the question: How many PowerSwitchTails can I hook to my Aduino? I want to control many lamps.

How many PowerSwitchTails can I hook to my Aduino?

How many digital pins do you have? How may shift registers do you have?

The answer, which a quick search would have given you, is an many as you want.