How to calculate torque required to rotate a shaft from rest, which is having four pair of gears like that of the clutch shaft in two wheelers

Are the 4 pairs of gears connected to anything? In other words - is this a gear set that's inteneded to make something go? Or are you asking how to calculate torque to spin up a shaft with gears and nothing attached?

Basically you need to define parameters such as mass and dimensions of the components, and expected rate of acceleration of the shaft. The relevant equations that can then be applied are and . As for frictional forces - that is very difficult to predict, as it is highly dependent on the gear design and lubricants. As a rule of thumb the gears in a typical automobile cause about a 15-20% loss in power between teh engine and the drive wheels. But to know for sure you will have to build it then test to see how it behaves.