My niece and nephew are joining us on holiday. Do I say You will need to arrange 'a flight' or 'flights' for yourself and James to arrive on Saturday morning, the 16th? This is assuming they will be travelling together on the same 'flight'.
Normally I would expect that if you have multiple flights in mind (i.e. potentially separate for each person), you would say "flights", but if you expect they only need one flight, you would say "a flight".
However, since in this context the word "flight" is shorthand for "a flight booking" or "an airplane ride ticket", then the plural may make sense. "You will need to arrange flights [bookings/tickets] for yourself and James..."