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.