Is the number of heart beats fixed in a human being? Is there any theory or theories on this? Can anybody help me?

No it isn´t. Otherwise, some babies dying in an early age must have had quite a rapid pulse.

 

If you meant that there´s an individual fixed number: Yes. You live, then you die. And inbetween your heart made a certain finite number of beats.

actualy that "theory" that the heart rate is directly related to lifespan was dissproven a very long time ago.

 

also an infants heart rate is near double that of a mature adult anyway, and bears no relation to SIDS.

That's just a myth perpetuated by lazy people to justify their lack of exercise! (exercise --> more heart beats --> early death etc)