Also depends on WHICH Robart Hinge points... there are at least 3 (I think 4 or 5) sizes.

With the ones that are appx 1.8 inch dia, and on a .40 size trainer I agree with the above numbers of hinges.

You NEED 2 on each moving surface no matter what size hinge point. In the case of the elevator halves, the 3rd each side is for safety, not because you need it to hold the elevator on. (if its a full span strip elevator like on a Kadet Sr... 4 total is plenty) On the ailerons you need the 4 because the ailerons would otherwise potentially flex between hinges.

Go to a .60 size plane... you need 4 on the rudder, and 5 on the ailerons (if full span strip ailerons)