How many times can you use a serial number?

I used my serial number on one computer and I'm trying to use it on the other computer. But, on the other computer, it keeps saying that the serial number is invalid. I checked my number to see if i typed it correctly, and its correct but it still wont work! So I'm wondering if you are allowed to use one serial number on two computers. (I'm triying to use it to download the Adobe Photoshop Elements 11 app)