Why does the Pilatus PC-12 go faster than the King Air C90?
The speed of an aircraft is not only dependent on the number of propellers. It is a function of (among other things),


Excess power available- The Pilatus PC-12 has more powerful engines compared to the King Air C90. The PW PT6 engine used in the Pilatus produces 895 kW, while (a different variant of) the same engine used in King Air produces 410 kW each. The power available for the PC-12 is more.
Propeller efficiency- The PC-12 uses a different propeller compared to the C90, which may have more efficiency at high speeds. Also, the PC-12 propeller diameter is more compared to the C90.
Aircraft design- The PC-12 flew around 30 years after the C90. The aircraft design has improved in many ways during the intervening years in aerodynamics, materials etc. These improvements can be used to increase the cruise speed.
Power to weight ratio- The PC-12 has a better power-to-weight ratio compared to the C90, which translates into better speed.


It is difficult to pinpoint any single reason for increased speed. However, the increased (cruise) speed in PC-12 can be explained using a combination of the above reasons.