A 4 litre tin will give you 2 coats (which is recommended) and you will also have some left over if you need to patch any areas in the future.
I have an Escort van and I used 3 1/2 litres.
A word of warning though, make sure all windows are open when you are driving after you protectakote it.
Its a solvent based paint and stinks to high heaven.
My van took 2-3 weeks for the smell to eventually go.
Matt