Each installation of WordPress, whether on a primary domain or subdomain (or subfolder) is counted as one “site” and therefore requires it’s own license.
If you have multiple subdomains/folders attached to the same domain you would require a separate license for each.
This is because in real terms a subdomain is still counted as a separate domain (it can be pointed at a different server) so we have to setup our licensing system in this way to keep things secure and organised and prevent potential abuse of the system