Probably 3 or 4 tins for the chassis.
I have just done the inside of both front wheel arches, the inside of the engine bay, and the tunnely bits inbetween (through which driveshafts and suspension arms) on my saab, and its used ~3/4 of a tin (a half litre one i think) It all had 2 coats.
I think i might stick to hammerite and underseal for the rest of it! - will be cheaper!
If you do use POR-15 then make sure you wear gloves and a long sleeved top, and dont shake it before using it (get loads of little bubbles in)