The questions is formatted this way for partial years. If you worked for the same employer, who offered a Health Insurance plan, all year you will indicate 12 months. If you only worked for part of the year it is possible that you were not offered a plan.
You will be able to answer this question precisely with the information from Form 1095-C. If you have not received this form, and your employer offers Health Insurance, you must request a copy of this form as soon as possible.