Short answer: Any size employer can offer an ICHRA. Employees (and reimbursed dependents) must have qualifying individual coverage or Medicare, and the employer can’t offer the same class of employees both an ICHRA and a traditional group plan.
An Individual Coverage HRA (ICHRA) is available to employers of any size. To be reimbursed, an employee must be enrolled in qualifying individual-market coverage or Medicare. Employers can offer an ICHRA to all employees or to defined classes (full-time, part-time, salaried, etc.), but they can’t give the same class a choice between an ICHRA and a traditional group plan, and class-size minimums apply when an ICHRA is combined with a group plan. Allowances can vary by class and by age/family size.