Answer:

65

In order to get the "13" wheel back to its original position, some multiple of 13 teeth must pass a fixed point.
In order to get the "6" wheel back to its original position, some multiple of 6 teeth must pass a fixed point.
In order to get the "5" wheel back to its original position, some multiple of 5 teeth must pass a fixed point.
Likewise, in order to get the "2" wheel back to its original position (neglecting the practical difficulties of getting a two-toothed wheel to work properly at all!), some multiple of 2 teeth must pass a fixed point.  Combining the requirements wheels 13, 6, 5, and 2, we find their Least Common Multiple (LCM).

LCM(13,6,5,2) = 390

So we need to turn wheel 6 enough times to make 390 teeth pass a fixed point.  Each turn of wheel 6 causes 6 teeth to pass a fixed point, so we need 390/6 = 65 turns of this wheel to bring all the wheels back to their original position.