As you know, the licences on WordPress themes such as Avada and WordPress plugins sold on the ThemeForest and CodeCanyon Market sites are for a single site use. However, it may be confusing when determing how many license you use in certain situations.
Here is the answer from Envato Market in December, 2017:
All of our licenses on the ThemeForest and CodeCanyon Market sites are for a single use. If you desire to use the item/template on multiple sites, then one or both of the following situations may apply:
1: If the item is used on one domain, and/or multiple subdomains, one license will suffice. For example:
2: However, if you are going to use the item on multiple domains, you will need to purchase a license for each domain. For example:
In general, with multi-site installations that are mapped to subdomains, a single license is fine because technically it's still only one installation and one entity. However, you'll need multiple licenses to use a multi-site installation with domain mapping because those would be technically different sites, and hence, different end products
As you see, you need only one license for multi-site installations that are mapped to subdomains. However, you will need multiple licnese to use a multi-site installion with domain mapping.
For your reference, you can use Elegant Themes' themes including Divi on unlimited sites for the licensed period (one year or lifetime) depending on plans.