As I understand, you get Plesk license for 30 domains and you act as a administrator of Plesk Panel.
In you Panel you see a list of service plans and you can create subscriptions to these plans.
Each plan has a "Domains" limit. If I remember correctly, it's set to 10 for "Default" service plan.
When you create a subscription derived from such plan, inside this subscription you can create 10 domains as maximum.
If you have a number of such subscriptions, you can create 10 domains in each of them as maximum, but total number of domains can't exceed 30.
If you want to create mroe then 10 domains inside each subscription, you must set higher limit in service plan.