This will actually depend on the use case. Licenses in general, apply to the end product you use them for. Usually, this end product will be a website with a specific topic. If you use sub-domains where the topic of the site is not identical or the general design is different, then these sub-domains represent own end products, thus you will need one license per sub-domain.
Only in cases where the topic and general design of your sub-domains match the purpose of your main site, it is fine to go with one license. In the latter case, you can contact us, and after review, we can enable your license for usage with sub-domains.