The main domain you use is considered as one license.
Once you decide to connect a subdomain, or use a totally new domain, this will be considered as another, one license.
Here is how it works:
domain.com can be your main domain. If you create multiple, unlimited tabs, for example: domain.com/bloginstall - this will still be considered as part of your one domain, although the pages can be completely different.
However, if you decide to create a subdomain, for example: blog.domain.com - this will be considered as a separate domain by the system (and once you set this up, it will be considered and cost as an extra license).
Depending on your iGloo Plan, you will generally either have 1 or 20 domains you can connect your pages to. For more see: https://support.sambakkerconsulting.com/hc/en-us/articles/115005341426-Where-can-I-purchase-more-domains-to-connect-to-my-account-