If they have been associated with your account for more than 90 days then you should be able to log into your account and remove the association via the 'manage devices' section : http://support.apple.com/kb/HT4627 - a maximum of 10 devices/computers can be associated at any one time, but the association should only happen if iTunes Match or automatic downloads are used, or if past purchases are redownloaded.
If you mean the number of computers that are authorised then you can either deauthorise a computer directly on it, or once every 12 months you can 'deauthorise all' : http://support.apple.com/kb/HT1420
Or do you mean something else ?
You can have a total ot 10 computer/devices associated to your account at any one time - as a computer also needs to be authorised on your account, and as at most 5 computers can be authorised, then only up to 5 of the 10 can be computers.
A computer or device only becomes associated when you use iTunes Match, automatic downloads or redownload past purchases on it - so you can have your content on more devices as long as you don't want to associate them.