That is a really hard question to answer. It depends on a number of factors, including the type of terrain you are modelling - winter, summer, dessert, rocky, how sparce you intend on scenicing your layout.
One trick I have learned is paint the base coat the same or similar colour as the ground foam you intend on using.
As far as I am concerned, you can never have enough scenery products and you will need additional stocks just in case you damage your layout so that you can perform repairs.
Woodland Scenics actually sells a kit that will scenic an 8x4 area, although I have never used it, but based on my experience with other Woodland Scenic products, you will probably need that and more to complete your project.
On another note, if you have a blender, some foam and some paint, you could always make your own ground foam products. This is an economical way to make scenery products, but make sure you don't use your wife's good blender!
I am sure that you will get quite a bit of feedback on this thread. I hope my comments help.
