Each purchase comes with a single-site license by default. That means one license per production website.
Envato purchases: your ThemeForest / CodeCanyon purchase code is your proof of license. Keep it — you'll use it to verify support and, for some themes, to register for updates.
ecomnius.com purchases: your license key arrives by email after checkout. It's tied to your order and activates updates and support.
Need the theme on more than one live site? Buy an additional license (one per site). Development, staging, and local copies of the same project usually don't need an extra license.