please tell me how many fault is generally occur in desil and gas generators in 1 mw range.

Many. All IC (Internal Combustion) engine driven generators have many systems and subsystems in common, such as; fuel, mechanical, electrical/control, governor, generator, excitation, etc., systems, A fault in any one of them will affect the proper operation of the entire system, regardless of the rating.

The best way to investigate this question is to do a FMEA (Failure Mode and Effect Analysis) on every component, system, and subsystem.

If you are looking for a reliability figure then then you will find such information on the web and/or from manufacturers of such equipment. Please be aware that these numbers are affected by the frequency and quality of the maintenance procedures, therefore it is difficult to give a general number applicable to everyone.