Your company is an Applicable Large Employer (ALE) if you have 50 or more full-time equivalent (FTE) employees in the prior calendar year. Only ALEs are subject to the ACA's employer mandate and affordability requirements, including the 4980H penalties.
If you have fewer than 50 FTEs, you're not subject to these requirements, though you can still offer CHOICE. The FTE calculation includes both full-time employees and part-time hours converted to FTE equivalents.