Do employers have to offer healthcare coverage to individuals who have completed a W-9 form using only his/her social security numbers?
If an employer has to offer coverage under the employer mandate they must offer coverage to 95% of full-time equivalents, there is no exception. So there is no exception for those who complete a W-9 form with only Social Security number. Simply the requirement to provide coverage is based on full-time equivalent employees and then substantially all (95%) full-time employees must be offered coverage starting in 2016 (although some larger firms have had to comply starting in 2015).
NOTE: There is a maximum of 90 days waiting period for new-hires in which they don't need to be offered coverage. Also employers have flexibility for accrediting hours and flexibility in terms of measuring full-time equivalent employees over time.
Learn more about the employer mandate.