Adobe allows you to authorize up to six computers and six mobile devices. Each time you authorize your device (even it's on the same device), it will count as one device. If you have reached the limit, you can contact Adobe customer support to tell them you have reached the quota limit and request them to increase the quota.
Please go to Adobe website here: http://www.adobe.com/products/digitaleditions/faq/.,
find the section called "CONTENT PORTABILITY". Under this section, you will find the questions "What is the maximum number of computers and devices that I can authorize?". Click on it and you will find the Adobe contact info.