You should be prepared to meet the most stringent requirements if necessary. Flights outside of the US generally have weight limits as well a limitation on the number of bags. It's always best to read your travel documents carefully and check with the airline or your travel agent if in doubt. I bought a large bag for a long trip to Asia, then got cold feet on actually using it after reading conflicting stories about weight restrictions on internal flights within China. All airlines allow passengers to carry on medically required devices, such as canes, walkers and crutches, and do not count such devices toward the number of carry-on bags allowed.
All airlines reserve the right to further restrict carry-on limites on crowded flights.