The number of rallies vary from club to club, as do the venues, some PC's have their own sites now.
You would use your own horse at all rallies - unless they were dismounted ones.
I haven't been involved with PC for a few years now but I think we used to pay about three pounds per rally to cover the cost of the Instructors fees. I imagine that this varies hugely from club to club. Many of our rallies were completely free.