For a complex itinerary, it is possible that multiple continuous tickets are issued at the same time under a single PNR. Usually you just need the first ticket number and the system should be able to call out the rest of it, nothing to worry about. Exact ticket number may be visible in the BA app and that could imply how the booking team split your itinerary from ticketing perspective. I have had a few bookings where 3 continuous tickets are initially issued.

If the ticket number are not continuous, then they are likely to be issued due to ticket reissuance following schedule changes or itinerary changes after initial booking. In any cases, as long as you see ticket number bound onto each segment you will be fine.