To send a fully authenticated email, each subdomain you want to send from will have to be added to Postmark individually.
DKIM and Return-Path are domain-specific, so each subdomain you add will have a unique DKIM and Return-Path records for Postmark. The separate DKIM and Return-Path records for your subdomains is the part that makes them unique so that their sending reputation is separate.
You can have as many subdomains added in Postmark as you need, there is no limit.
Sending from a subdomain would require you to verify the subdomain with DKIM and a custom Return-Path.