Hi,
Thanks for the interest in our plugins.
The license if for automatic updates in your site and is not needed for the plugins to work.
So generally you add the license where you want to make the automatic updates. We have this guide on using the license on a live and dev site:
https://theeventscalendar.com/knowledgebase/using-one-license-for-your-live-and-dev-sites/
As for the amount of license keys you should purchase based on the number of live sites and not include staging, dev, or sandbox sites in that number.
https://theeventscalendar.com/knowledgebase-category/purchase-install/
Let me know if you have any follow up questions.
Thanks