Journey Mapping Project best practices

Looking for suggestions on process best practices:

In order to understand how clients (e.g. insurance carriers) subscribe to services from a particular company, I need to create a journey map to showcase the different phases from awareness, interest,..,loyalty, leave, return.

Is the best practice to start by interviewing 5-7 participants (prospective clients)? How do we converge all the data from individual personas to the main persona?