Welcome to the Apple Community hmartin 14.
It might be that you have too many computers registered, rather than too many devices.
You can't de-authorise just the computers you no longer have access to.
Your only option is to de-authorise all your computers and then re-authorise those you wish to be authorised. You can do this by logging into your account settings in iTunes on your computer (Store > View My ID) and selecting the option to de-authorise all.
Please note however, you are only permitted to do this once in each 12 month period, therefore you would be better de-authorising your computers before you lose access to them. you can do this from the 'Store' menu in iTunes.
You can see how many associated devices you have in the same account screen.