MGNLSITE-105 and MULTISITE-122 brings new SiteRegistry, unfortunately they are not limiting number of allowed sites configured in the license.

Developer notes:
While implementing the new registry, limitation for number of registered sites was removed. This check was previously done in MultiSiteRegistry#register and MultiSiteRegistry #unregisterAndRegister methods, but these are deprecated and not used with new ConfigurationSource mechanism. Registry doesn't hold the map of registered definition, but delegates into ConfigurationSource(every source holds own RegistryMap), so during the registration it is not possible to know how many definitions are registered by the other ConfigurationSources.
I also tried to implement SiteCountValdiator and add it via info.magnolia.config.registry.AbstractRegistry#addValidator, but that marked all definitions as problematic, but still registered them.