In iTunes, under the "Store" menu, you can select "Deauthorize this computer". That will remove that computer from your iTunes account and free up a spot for the new computer.
If you have already sold or wiped your previous computers, you can log into your iTunes account from the Store menu in iTunes by selecting "View My Account xxxx@xxxx". Once you are logged in, about 1/4 the way down the page it will tell you that you have "xx computers are authorized......". Beside that you have the option to deauthorize all of the connected devices. That is the only option, you will have to deauthorize all and then set up the ones you want to use.
I think 10 is the maximum number of devices that can sync to one iTunes account.
You can read this article about deauthorizing computers. You can only have five computers authorized for iTunes content at any one time, so if you have exceeded that, you need to deauthorize at least one of them.
You can also only have up to 10 total devices associated for the same Apple ID. In case that is the issue, you can remove associated devices in your iTunes account if necessary,