I'd say it depends on the round ... for a .308, I'd say 10 cases are plenty, if you plan on shooting it. The .308 brass will be too easy to come by to let it get in the way of getting the dies on order
If on the other hand, you are starting with some unknown wildcat, I'd buffer at least 100 and have a viable source for more. I just bought a Sig P225 and ordered the 9mm dies the same day I ordered the pistol (zero brass in inventory) ... I know I will be reloading light loads for my wife, so it wasn't an issue.
Just my opinion ... Saands