Depending on which article or newsletter you see, the number of full-time and full-time equivalent employees that subjects an employer to the ACA’s employer responsibility requirements is either 50 or more than 50. Hmm, it can’t be both! So, this answer is correct?

The answer is 50! Here is the answer straight from the IRS:
To be subject to the Employer Shared Responsibility provisions for a calendar year, an employer must have employed during the previous calendar year at least 50 full-time employees or a combination of full-time and part-time employees that equals at least 50.