HOW MANY MOSFET CAN BE OPERATED USING ARDUINO UNO AND ARDUINO MEGA?
How big a budget do you have?
Or perhaps more importantly what do you want to actually do?
ANANDJOSHI555:
HOW MANY MOSFET CAN BE OPERATED USING ARDUINO UNO AND ARDUINO MEGA?
Well now, hang on there, pard'ner, and have a look at this:
http://forum.arduino.cc/index.php?topic=149015.0
That'll help you, to help us, help you 
ANANDJOSHI555:
HOW MANY MOSFET CAN BE OPERATED USING ARDUINO UNO AND ARDUINO MEGA?
With some additional hardware like shift registers or I2C IO expanders - unlimited. However - more mosfets you have more time will be required to serve all of them.