Seriously, if I needed twenty or thirty shots of every car that were usable in every session, you'd have to go for reasonably safe.
You don't - most people only want one or two shots. I got 14 from the Frozenspeed guys at the 'ring. It meant I could select myself, which (for me) was simply great - probably 5-7 of them didn't tick my boxes, but with cars going round on track you have the full day to shoot them and (if you set expectations right) folks are mostly happy with this sort of 'all in' deal to pull the photos down online later, or grab a memory stick at the end of the day. We pulled the photos down at a wireless hotspot the day after.
As I suggested previously - switch modes every so often - the togs should know whether the day's open pitlane, or sessioned, so should know when to switch between shooting modes - if you don't know you're not doing your job effectively!
Trackside photographers seem to be operating in a very 'film-based' mode still, as described a couple of posts ago. Develop at lunchtime, sell before people go home. I understand that the best way of selling is on the day, but I've not seen a UK motorsport tog pop up at a circuit briefing and make the sort of Frozenspeed offer before everything gets going - it's different and it works. If I bought and paid for all the pics at the briefing it was 50Euros. If I did it after the event it would have been 100Euros - not a tough call as someone who hadn't been out there for a couple of years.
Only annoying thing for us was we had an off at 09:30 and didn't get back out until 13:00hrs - they were shooting in the morning, so we only got photos from the 08:00-09:30! Still, that was a risk I was willing to take. The step on from that is that next time someone could make some money by being out shooting in the afternoon, when the official togs are holed up downloading the images on to memory sticks!