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.
Multiple sub-domains as multiple end products
Suppose you do a website where you present different branches of your business in different cities or countries. Thus, you create two different sub-domains: brancha.my-business.com and branchb.my-business.com. If the purpose of each of the sub-domain sites is a bit different or the general design is different, you have created different end products. So in this case you need one license per sub-domain.
Suppose you do a website for a retailer of apples and a retailer of pears, both hosted on your main domain. Thus, you create two different sub-domains: apple-retailer.my-grocery-hosting.com and pear-retailer.my-grocery-hosting.com. Your end products here clearly are the websites for each of the merchants, so in this case you need one license per sub-domain.
Multiple sub-domains in one end product
Suppose you do a website where you present different departments of the same local company. Thus, you create two different sub-domains: sales.my-business.com and customercare.my-business.com. If the departments are presented in a way that uses the same general design as your main site, they count towards a single end product. So in this case you don’t need extra licenses per sub-domain.
Suppose you do a website for a grocery store and among other goods, you sell apples and pears. To show your supplies, you create two different sub-domains: apples.my-grocery-store.com and pears.my-grocery-store.com. Your end product clearly is the website for the grocery store, so you only need one license.