Dear Umar,
The applicability of the Industrial Disputes Act,1947 is not restricted by the number of workmen/employees employed in an industrial establishment/undertaking. Of course, there are certain restrictions with reference to the number of the employees in respect of certain matters viz., Lay-off and Retrenchment of employees and closure of the establishment and formation of committees viz., Works Committee and Grievance Settlement Committee ; certain classification-based restrictions in matters of strikes and lock-outs.
I would like to request you to go through the relevant chapters/sections of the Act for further details in this regard.