It’s one ticket per registration. So if there are 2 registrations completed, that’s 2 tickets. One ticket can allow entry for many datetimes if there are multiple datetimes attached to the ticket.
So it sounds like you may not need to cancel any registrations, the way forward may only involve making sure that the tickets are attached to the correct datetimes.
If you do need to cancel some registrations you can get the name of the ticket (set in the name field of the event editor), then do a search for that ticket name in the Event Espresso > Registrations page and that will list all registrations for that ticket.