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 employees in respect of certain matters, such as lay-off and retrenchment of employees, closure of the establishment, and formation of committees like 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.