Dear Ganesh,
The very question you have raised is one that always arises in the inquisitive mind of a person who gives serious study to labor laws and their applicability based on the number of people employed in certain types of industries and certain types of employment benefits, as mentioned by Dr. Sivakumar.
Practical difficulties in implementation
The primary reasons appear to me to be the practical difficulties due to:
1) The implementation of technical standards involving the safety and health of the people who are directly associated with the processes carried out therein and the environmental impact on the surroundings where such industries are located.
2) The nature of certain employment benefits like payment of bonus, gratuity, enrollment into social security schemes like provident fund, and insurance against injuries arising out of employment accidents, which require constant adherence to rules and regulations and upkeep of proper professional accounting, which might be very difficult for a small employer.
3) The number criterion could have been, as opined by Dr. Sivakumar, adopted from the Factories Act, which is the mother of all labor laws of the past two centuries. The reasons should be ease of compliance and effective enforcement.
Impact of labor laws on smaller establishments
This explanation may sound inadequate as there is the possibility of employees of other establishments being deprived of the rights and benefits available to the employees of large establishments. If we trace the general origin of laws, we will find it emanating from the customs and practices of orderly societies to regulate the conditions of living and cordial relationships. Therefore, the main purpose of the enactment of laws is to prevent possible mischief likely to be dictated by unscrupulous tendencies. The relationship of employment, i.e., master and servant, is not an exception. This relationship naturally turned out to be an agreement earlier specifying the mutual rights and obligations of the master and servant, slowly metamorphosing into what we call now a contract of employment. The advent of the industrial revolution and the emergence of joint-stock companies coupled with the trade union movement necessitated the passing of numerous labor laws pertaining to establishment-specific, employment condition-specific, and industrial relations-specific. The impacts of the IT revolution and LPG brought about a paradigm shift in the employment pattern, as a result of which we have new classes of employees like gig workers, platform workers, etc. However, you may note that labor laws governing certain basic and universal conditions of employment like working hours, leave benefits, minimum wages, tenure of employment, and redress of employment grievances do not rest on the number criterion for applicability.
Provisions for small employers
You should also note that a saving clause has been included in such number-based laws to ensure continued application even if the number of employees falls below the minimum later, and enabling provisions are there to reduce the number to extend the application. In technical labor like the Factories Act, 1948, there is a provision under section 85 to notify industries engaged in certain manufacturing processes as factories for certain purposes of the Act, even if they do not have the minimum threshold number of employees. There is no legal bar on a small employer to bestow the benefits of bonus, gratuity, etc., to his employees, and in fact, such things happen depending on their cordial relationship and customary practices as in the case of puja bonus to employees.
Therefore, the number of employees in an establishment as a requirement for the applicability of any particular labor law is based on the principles of technical feasibility, professional accounting system, the nature of the service conditions and employment benefits, and so on. That's why it is 10, 20, 50, 100, or 300, and even the number game comes into play in respect of certain conditions like layoff, retrenchment, etc., in laws which are not restricted by the number of employees principle.