The bikes belonging to Dave and Marty
To determine the number of bikes belonging to Dave and Marty, we would require more specific information about the bikes. If you are asking for the total number of bikes between Dave and Marty, we would need to know the individual number of bikes each of them owns.
If you have that information, you can simply add the number of bikes owned by Dave to the number of bikes owned by Marty to find the total number of bikes.
For example, if Dave has 3 bikes and Marty has 2 bikes, then the total number of bikes belonging to Dave and Marty would be 3 + 2 = 5 bikes.