Sir(s),
1. Trading business firms are generally called "shops" and are covered/coverable under ESI Act, 1948 under section 1(5) of said Act and rules/regulations framed thereunder.
2. Before, however, action for getting Code Number under said Act is initiated, you can see and confirm whether the appropriate government has extended the provisions of said Act under above provisions of the Act. You can confirm the same from nearest ESIC Branch Office/Regional/Sub/Div/Office.
3. At present most of the appropriate Govts. have reduced the threshold of coverage of such "shops" in their States to 10 or more persons. However, in few States like Maharashtra, it is perhaps 20 or more persons.
4. You can also confirm before getting code number from ESIC whether the area where your said "shop" is located is implemented area under section 1(3) of said Act or not.