How to track of which envelope is associated with which a user when sending the same document to each client for e-signature?

Most signNow users build a fully functional integration.

Integration such as Salesforce, CRM and other products that integrate signNow into their system track it either through an Envelope Custom Field (with their internal application record information) or they document the envelopeId responses and store them in their application.

Either way will work out just the same, it's all on how you want your integration to work. Here is a quick sample call on how to create an envelope from a template with a text custom field