Dear Ambuj,
In general, if you are a contractor & is employing 20 or more workers then you are to obtain contract Labour Licence from the Licensing Officer of the area. Similarly, if any of management is employing 20 or more contract labourers through one or more contractors then he is required to have Registration Certificate under CLRA Act from the Registering Officer of the area.. Appropriate Govt. can increase or decrease the no. of contract labour for its applicability. You can also go through this Act to have some basic knowledge on its applicability.
R N KHOLA