Does there have to be a certain amount of employees to have disability insurance?

In the US, California, Hawaii, New Jersey, New York, and Rhode Island impose mandatory state disability insurance programs for employees. The purpose of the programs is to provide some protection against wage loss caused by short-term non-work-related disabilities. The insurance premium is submitted to the insurer by the employer but paid either jointly by the employer and the employee, or entirely by the employer, depending on the employer's good will. There are some limits to what the employee may be required to contribute by the employer. This insurance is in addition to two well-known government disability programs: Worker's Compensation and Social Security. Employees' contributions are federal tax-deductible.

Simple answer: No. Group Disability Insurance is not like Group Health Insurance -- and all the ERISA regulations that control how this employee benefit works. With Group Disability Insurance, an employer can "carve out" a select group of employees -- meaning the employer can create a "plan for just one employee (himself!)".

An employer can also offer a contributory insurance plan, in which case the employee will contribute a certain percentage of premium. Or the employer can choose to offer a voluntary plan, where the employees enroll on their own accord and pay full premium.