Settings:
Calculate Number Registrants Based on Tickets Quantity: yes
Prevent Duplicate Registration: yes
Behavior: Registrant can still select multiple tickets with free event and "Ticket Types" enabled. Number of registrations is also displayed under Registrants as > 1 for the registrant.
I understand that a second registration after completion is not possible but I would like to prevent registrants to book more than 1 ticket.
Is there any way to do that?
No, it is not possible. Prevent Duplicate Registration, right now, can only be used to prevent one user from registering for an event two times only. It does not control registering for ticket types as you described
