Q: i have an itunesmatch subscription. 3 units are associated to the account . i try to add my computer . tells me i gone over 5 units allowed. what is the problem

Whilst there is a limit of 10 devices for iTunes match, there is also a limit of 5 computers for other iTunes things, and this is probably what you have come up against.

Do you have more computers that aren't associated with iTunes match?

If you go into the iTunes store and click on your account ID in the top right corner then check under the iTunes In The Cloud section for the number of devices associated. You can then select manage devices to see exactly what is associated.

I suspect you may find there are 5 computers there.

Your Apple ID can have up to 10 devices and computers (combined) associated with it.

Like Automatic Downloads, when you subscribe to or enable iTunes Match, your device or computer will be associated with your Apple ID.

What I would do is do a reset of Itunes authorizations. If you had iTunes on an old computer, and never deactivated it, then that is your problem. The best thing to do deauthorize them all, and the reauthorize the computers that you currently use. This happens a lot when someone's old computer dies or is thrown out before they realize they have to deauthorize it and now cant do it without having that device physically. Also happens when you reformat the hard drive :-). I bet you will find that your over the limit because of old computers.

To deauthorize all computers associated with your account

1. Click iTunes Store in the Source pane.
2. If you’re not signed in to the store, click the Account button, then enter your account name and password.
3. Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
4. In the Account Information window, click Deauthorize All.

After that, then go back into each computer and authorize it the same way.

http://support.apple.com/kb/HT4914