I've run up to 30 freight cars behind my BigBoy on O54-O72 curves and mostly flat track. My cars are mostly postwar Lionel which don't roll very freely, even after cleaning and lube. The loco could pull more but some couplers would pull apart with more load. If I had newer cars that rolled more freely I'm sure it could pull a longer train.
I've pulled 40+ cars on part of my HO layout even with 15" radius curves and some steep grades.