iTunes Authorization Issue



I've run into a bit of a problem with iTunes. When I first got my Mac, I authorized it to play music I download from the iTunes music store. After a few months, my computer broke down and it had to go into a tech shop several times to get its memory replaced. Each time the computer came back, I needed to give a new authorization.

Long story short, I have used up 4/5 of my account authorizations on the same computer. I want to deauthorize my Mac 3 times so that, say, I can allow an iPod or two to play my music. However, I cannot find a place to do this. Does anyone know what I can do?