What is derating factor in simple word, how I can calculate this factor for cable, give me formula if any

Derating cable means to lower the ampacity of a conductor due to having multiple current carrying conductors in a raceway due to the heat that can be created.

There is no specific formula. Derating will be determined by codes and standards.

In the USA< the National Electric Code is the standard that dictates that more than 4 current carrying conductors in a race way will need to be derated, beginning with 80%. As the quantity of conductors increase, the ampacity decreases.

You need to refer to the code or standard that has jurisdiction of your country, state, etc. for derating values.