The twin word denotes two cylinders and the arrangement in some shape defines that it's a V or parallel twin or even L in the case of Ducati. The most common V-twin is the shape made by two cylinders in some angle between 42 to 90 degree. Harley's use 45 degree making a perfect V whereas Ducati uses 90 degrees, making the L shape and hence naming it as L-Twin. Here, one of the two cylinders is parallel to the ground. The parallel twin cylinder engine has two cylinders equally parallel to each other like two different engines glued together.
* Parallel Twin
The inline 4 engine has 4 cylinders and as the name says, they are in a single line. The size of the cylinder denotes the cc of a bike and while a 998 cc bike can be classified into 2 cylinder with 499 cc of each or 249.5 cc of 4 cylinder each. Every brand prefers it's own engine design and Ducati uses L-Twin design with each cylinder measuring 642.5 cc for the expensive 1299 Panigale S.
* V-Twin from KTM
Rest of the power figures depend upon the technology used on the bike. Fuel injection, slipper clutch, advanced gearbox and other such equipment helps the bike maintain its power from the engine and deliver it in the best form to the wheels.
The first power determining factor is the size of the engine and then comes the technologies mixed to its production. The number of cylinders usually determine the riding feel at low rpm's as 4 small cylinders produce lesser torque from each one, producing lesser vibrations while bigger ones produce more vibrations.
* 3-Cylinder Engine
KTM and Ducati, along with Triumph use 2 and 3 cylinder engine with best technology from the world to make them more powerful and smooth as compared to inline 4 from many brands like Benelli and Kawasaki. KTM's single cylinder RC390 is very fast while the same acceleration is produced by a twin cylinder engine of the same size. The difference lies in just the low and high end torque distribution where the twin one gets the benefit.