Insurance - since Brunty isn't actually a track day, (it's a private event for charity on a proving ground, not a circuit, there's no racing, no timing, no helmets), then cover may be available from your normal insurers. Certainly some of us have been successful with this in the past. Personally I've had mixed results, but it's worth a try for free.
Alternatively, track-day insurance can be purchased from the likes of Moris, Adrian Flux and a load of other operators.
I have a suspicion that many people don't bother - it's pretty low risk, huge run-off areas, and on the whole people are very well behaved.
The number of laps people do varies a great deal. Really, you can do as many or as few as you like, but if you want to come with the idea of only doing a couple of laps, then that's absolutely fine but please let us know, as that will help with scheduling the cars through the day.
The cars aren't track cars - they need to be rotated to allow primarily brake recovery time, that's the weakest point on them. In general, doing a session of about 30 minutes will give you about 6-8 laps, with loading/unloading pax, and the time through the pit/gate area. Good scheme then to rest the car, help out with marshalling perhaps, see the rest of the show, maybe have a passenger ride in another car (there are some stunning rides on offer), and then do another slot if you are happy with it.