How much paint

How much primer, undercoat and top coat will I need for a 4" agricultural engine

my mate has painted a showmans not a STW but only used 1 litre and still done 3-4 coats on it, ive only bought 1 litre and im building a agricultural to

Thanks, is that 1 litre of each, primer, undercoat and top ?

I bought a 1litre pot each of undercoat and enamel , but over the course of the build had to buy more as the the paint began to solidify. so you may prefer to buy smaller pots.
Ian