Here is the info from most common code violations in Denver:
"Too many unrelated people
Any number of persons related by blood, marriage or adoption may live together in a single-family residence. In a single unit zone district, two unrelated persons per household are allowed, and with a home occupation permit, an additional unrelated person is allowed. In a multi-unit zone district, four unrelated persons per household are allowed, and with a home occupation permit, two additional unrelated persons are allowed." link below
https://www.denvergov.org/developmentservices/Deve...
As for enforcement, they usually only enforce if they become aware because of complaints from neighbors. Then they will enforce it. We have a single family with 4 bedrooms that we had rented to 4 students for quite a while. Unfortunately, a woman moved into the neighborhood who has decided she doesn't like students and seems to be doing everything she can to get rid of the student rentals in the neighborhood.
We were informed that we were only allowed 2 unrelated until we filed for the home occupation permit to allow a 3rd. The permit is automatically granted if filed. Now we only have 3 students in there and are trying to figure out how to move forward. They can't afford the house with only 3 of them. Lease is up next month and we will probably try to find a family that can afford the space.
Not sure if this helps, but we had never heard of the problem until it we had to deal with it ourselves. Our property management company had told us it was not an issue and not to worry about it. They were wrong.