Hello,
Actually it works the way you've described - You need unique license key for each domain, including sub-domains. However it is no problem - the sites in described scenario have the same content (well, showing it different way on each sub-domain), so you can run it with
1 web site license (please don't mess it with license key itself). Therefore, in case you have bought this license, you can write at support@kentico.com or sales@kentico.com and we will provide you with appropriate license key in return.
Best regards
Ondrej Vasil