Two per square foot should be fine. I use the square foot gardening method, and there is a forum on GW dedicated to it. Also, you can go to gardener's supply website and they have a planner that will show you how many plants per square foot to have.

Here is a link that might be useful: Kitchen planner from gardener's supply

have you checked out square foot gardening?

even if you don't use all their recommendations, their design for spacing should help you

Here is a link that might be useful: square foot gardening