I believe your issue can be easily solved - have supervisors wear a distinctive color from a non-supervisor. For example let's say it's yellow.
OK so now you have yellow wearing supervisors and everyone else. To resolve the issue you have described acquire a month or so of footage, with labels per minute describing how many yellow wearing supervisors and how many people (in total) there are.
So the data you have is:
1. Yellow wearing supervisors
2. Total amount of workers on the floor
Then with this data you can train a network to do what you're describing pretty easily. Assuming there are a lot of workers on the floor, trying to do person detection or face detection would require too much data. Just have a uniform enforced and train on the colors/presence.