Simply de-authorize those computers that you no longer have. Apple only allows 5 computers on the same account. Do this:
Click the iTunes Store in iTunes, sign in to your Apple ID, click the name of your Apple ID, and select Account. From here, you can click the Deauthorize All button next to Computer Authorizations. This button will only appear if you have more than one computer authorized. After doing that, don't forget to authorize the computer(s) you want to use.