I imagine that the Rotax 912 powers a lot of one offs and kit-set aircraft types because it is (I am presuming here!) one of the more reliable engines in this class, and has been in production for quite a long time. The fact that it has been produced in some numbers (does anybody know how many?), and used by so many "types" is probably quite noteworthy, but the financial investment in each engine is comparatively tiny when compared to the investment in each big reciprocating engine. That is why I think the question of greatest number of aircraft types using any particular aero engine (piston or turbine) is of rather less consequence than the rather simpler question (number of each engine type actually manufactured).