Please refer to the IRS documentation to determine if your company is considered an ALE:
IRS: Determining if an Employer is an Applicable Large Employer
The short answer is that your company is considered an ALE if your company has an average of 50+ Full-Time employees for the prior calendar year.
A Full-Time employee is any employee who works over 30+ hours per week, regardless of their employment status.
If you have a Part-Time employee who is working over 30+ hours per week on average for a known period of time, that employee is considered to be a full-time employee by the IRS.
If the average number of Full-Time Employees is 50 or more for the 2017 calendar year, then for 2018 you are considered an ALE and you should be tracking your ACA compliance.
If you have more questions, please contact firstname.lastname@example.org.