Well, at most you can get 61 19" cars plus your loco... But most G-scale trains I have seen run with 3-6 cars plus caboose. One "extra long" train I saw this past weekend at a garden RR was 8 cars plus caboose, but it was double headed. If the track is level, you are probably reaching the pulling capacity of your loco at 8-10 cars anyway.
Do a test and let us know what you find out!
Andrew