The only airline I can think of that's run as many fleet types as say this: http://ae31.airline-...e3r2&player=297 is Aeroflot of the USSR. And most of Aeroflot's aircraft back then were in more or less a military/governmental role, not a commercial one. Thanks to the free market, airlines are doing all they can to keep their fleet types under control. However, one might say that there are exceptions, and there are. For example, Delta Airlines, the world's largest airline in terms of fleet numbers, operates 20 different individual aircraft types (ie: MD-88, MD-90, 737-800...). The airline I used mentioned above operates 44. The largest airline in RDelta so far operates 11, with a fleet of 160 aircraft.
To me, this is silly, and personally I'd love to see increased penalties for operating many different types. Because logically, if I operate DC-10s, MD-11s, L-1011s, 747s, 767s, 777s and IL-96s for my intercontinental flights, my maintenance should be stupidly high for these aircraft due to vastly different engine components and whatnot. Crew training costs should be high because none of these aircraft have similar cockpit layouts and flight characteristics. But, alas, this limit does not exist in AE.