Your manufacturing facility will come under Factories Act and for that the leave rules as provided in the Factories Act will be applicable. As such every employee who has worked for 240 days in the preceding year (normally the leave will be credited in January every year based on the attendance of the preceding year) should be given one day for every 20 days worked. For deciding the eligibility you should take the paid holidays and leave days, lay off days etc, means the paid days, whereas for deciding the number of leaves to be credited or given to each employee his physical presence in the factory should only be considered.
If your trading activity is carried out in a premises separate from the factory, the employees in that office will get leave as per Tamil Nadu Shops and Commercial Establishments Act. Accordingly, every employee will get 12 days EL, 12 days Sl and 12 days CL.
You can have more number of leaves but you cannot contract it out by means of any clause in the appointment order.