Is it common to have a different preceptor consistently?
I trained in the EMS system about 70 miles west of where I currently work FT. We had 1 preceptor, and with rare exception, 2, for our entire duration of field time. Some students needed 2 due to work schedules. I started with 1, but he was the most senior paramedic at his service, and took a month long vacation out of the country, past halfway of my program, so I got a different preceptor to finish with.
The system where I work lets the paramedic students ride with any preceptor that has been approved by 1) the college 2) the system 3) the service. You could easily have 20+ preceptors during your time, and some of the students do.
I prefer the limited number approach. I think it's good to have 1 person tracking your progress. I'm a preceptor in both systems, and the one where I trained really requires preceptor buy in. They even pay you by giving you free credit hours at their college.