299US$ Upgrade for Pro is for 1 site or any domain with my License I'm not sure 299US$ for Upgrading to Pro License is for 1 domain on upgrading or any domain with my account (Craft CMS Member).
It is per domain (excluding sub-domains), i.e. for one domain / website.

domain.com and subdomain.domain.com can both run off of the same license.

More info on how that is all enforced.