As far as I know, the only time the subscription process is supposed to ask for a serial number is if you enter the information that you are asking for the special price for existing software... is that what you are doing?
Cloud Plans https://creative.adobe.com/plans
-and subscription terms http://www.adobe.com/misc/subscription_terms.html
http://helpx.adobe.com/creative-cloud/help/install-apps.html to install or uninstall
Cloud Getting Started https://helpx.adobe.com/creative-cloud.html