I found that there are some different opinions on which license of Themeforest WordPress themes is required for multiple subdomains such as multisites. For subdomains it's possible to have one license if they are on one domain.
Here is the answer from Envato Market:
Therefore, one license is needed for "www.domain.com/siteOne", "siteTwo.domain.com" and "siteThree.domain.com." However, for multiple domains such as "www.domain-siteOne.com" and "www.domain-siteTwo.com", you will need to purchase a License for each domain.