If more courses are run, is the start date based chronologically on when people are interviewed and accepted or merit of the applicants?
Unlikely.
The Qlink hold file is reasonably large and some applicants have been on it for up to two years.
Obviously, the hold file contains applicants who've been successful in all the interview stages but there is no current position for them. These pilots will have differing qualifications, degrees, hours, and endorsements.
When the a ground school is generated, the company may have preferences for pilots with certain qualifications or endorsements, based on the projected needs at that time. For example, the company may want pilots with (or without) an ATPL.
Based on whatever the needs are at the time, the company will contact those applicants preferentially over others, regardless of when they made it onto the hold file.
DIVOSH!
Disclaimer: The above is my opinion only.