One license can be applied in a multisite installation. The only requirement is that the sub-domains, sub-folders of this site should belong to one single end product.
Create a domain (website.com), which becomes be your primary (parent) domain. Then attach subdomains (sub-domain.website.com) or subfolders (website.com/sub-domain) and use the same license key to register the Motors instances on the primary domain.
Or use WordPress Multisite feature to create Motors theme instances and register them using one license key.