This question is answered by the IRS: https://www.irs.gov/affordable-care-act/employers/questions-and-answers-on-employer-shared-responsibility-provisions-under-the-affordable-care-act
An employer determines if it is an ALE for a current calendar year based on its number of full-time employees (including full-time equivalent employees) during the prior calendar year. If an employer hires additional employees, including some part-time employees, during the current calendar year, the employer will take those employees into account when determining if it is an ALE for the next calendar year.