Re: How many cans of paint?
If one does the math, then the surface area that you're talking about painting works out to roughly 600sqft. So then for 3 coats that's 1,800 sqft. And at 450-500sqft/gallon the amount of paint seems correct, knock on wood.
That said, ask him to break down the numbers for you. He IS the expert in this equation. And you might discretely ask another pro or two to run the numbers for you as well. Doing so such that everyone involved, including your painter, knows that you're just trying to confirm that everything's being done correctly.
Much like you'd get a 2nd opinon from an MD prior to a major proceedure. With your primary MD knowing that you're doing as much.
The Uncommon Thing, The Hard Thing, The Important Thing (in Life): Making Promises to Yourself, And Keeping Them.