How does sealed aircondition room get oxygen so we not suffocated?

A person expends about 2000 calories in a day so in a night about 667. Now most of that is received from sugar at about 4 calories per gram so about 167 grams. About 40% of that is actually carbon atoms so about 67 grams of carbon is released by a human in 8 hours. This means about 178 grams of oxygen is used up to make CO2. If we assume air is mostly made of similar weight particles, a 27 cubic meter room will have about 6.8kg of oxygen in it. The reduction in oxygen by you being in it is thus less than 3%. I'd imagine that is really insignificant and also a 27 cubic meter room is pretty small so the number would be lower if you are in a decent hotel room.

In addition to this, all HVAC systems have a damper that allows outside air in. Older systems use a damper with a fixed % opening that closes off if the outside air gets too cold and may freeze the coils in the winter. Modern systems use Co2 sensors, many built into the thermostats, that regulate the amt of fresh air intake. All schools in my district use these sensors. I worked there for a few years, and we could see the Co2 levels spike when the kids came into the classrooms. The HVAC would run continuously during the day with the heat or cooling coils turning on and off as needed, and the outside air damper exchanging the air as necessary.