If your server is licensed, stored or non-running instances of Windows Server (and other Microsoft servers) do not need their own licenses (surprise!). The usage rights of your server license will permit you to store any number of instances under each license.
This also includes any of your storage media (SAN) – but you must be sure they are non-running, otherwise you will be considered out of compliance, according to your Microsoft EULA. As long as your physical server is licensed, there is no limit to the number of instances you can store under that license.