Hi Nigel,
Thank you for your purchase.

As we are using Envato as the marketplace for selling the product, their licensing policies apply: https://codecanyon.net/licenses/terms/regular

Their policy in general is: one license is per one domain or project.

If your multiple sites are sub-sites of the same domain, and apply to the same project, you can use one license.

If these are installed on different domains, and are different projects, you would need a separate license per each.

Best regards.