Good evening Jatinder and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
Technically speaking, each subdomain counts as a domain for the purpose of licensing.
For that purpose, people usually opt for a multisite license (which gives the added perk of being able to update from wp-admin).
You can read all about it here: https://theeventscalendar.com/knowledgebase/multisite-license/
Let me know if that helps.
Have a great day!
Geoff B.