If your painting the whole van including the roof and inside, 5 or 6 ltrs is probably right, as cellulose doesn’t cover as well as the newer 2 pack system.
I have used 2ltrs of 2 pack straight colour on the roof and inside already and will probably use 2 lrts of basecoat and 2ltrs of 2 pack laquer on the rest of the van when I get round to spraying the body next month.
Estimating paint coverage isn’t easy on something youve’ never painted before. 🙁