In Texas, the state requires that each teacher have a minimum of 45 minutes of duty-free prep time every day (or 225 minutes a week, divided differently). On my high school campus, we have negotiated to have 2 prep periods every day, but we may be assigned a duty during one of the prep periods. This usually happens a few months before our state-mandated testing, when students are pulled out of electives and specials to receive extra tutoring from math/science/english teachers.
As far as I know, there are no regulations regarding the number of different preps one teacher can have. In my department, one teacher has 8 different preps and another has 5. I have 2.