If your business is in a non-construction industry, once you have 4 or more employees, yes it is required by law.
If you are in the construction industry, 1 or more employees, including the owner of the business who are corporate officers or Limited Liability Company (LLC) members.
If you are a contractor, you are required to make certain that all sub-contractors have the required Workers' Compensation Insurance before they begin work on a project. If the sub-contractor does not have Workers' Compensation for its employees, those workers become the employees of the contractor. If an injury occurs, the contractor is responsible for paying the benefits for the work related injury, illness or fatality.