While this reply is probably more from experience in the NT than anything else, you would have to define the level of "custody" as well.
Ive flown illlegal immigrants in custody, 6 warders and 10 illegals.
Kids in custody, usually an officer, a couple of kids and a FAC's person seem to turn up.
Then there is the violent crime lot... 1 guard at each exit, and one per prisoner through to the really nasty ones where there is two officers per prisoner. The prisoner is usually in leg straps and cuffs by then.
Psychotics (unstable violent offenders) with section 9 orders under the mental health act, sometimes end up on the Medivac planes, fully restrained and sometimes sedated for transport to a mental health unit.
I learned a couple of things... No, I wasnt allowed to cuff the nasty ones to the plane, most sleep or doze during the flight, and most of them were terrified of flying.
The custody officers always looked out for me though.