How many main parts of a computer?

It depends how far down the rabbit hole you wish to travel. At the top most level the basic computer has a motherboard which houses the basic components of a computer, you can have the CPU, Memory, BiOS, GPU, Network card, and a soundcard. Along with alot of other components. Not all of the above are necessary and in all computers. Even lower down the rabbit hold you can get into the sub components such as the MTU and others. But assuming CPU, Memory, BiOS, GPU, Network card, and a soundcard are the core components should stter you wrong.