How expensive is it?
How much can you afford to spend?
I do know of (not personally) people that have a pattern all picked out and buy practically to the inch. It would probably be a lot tidier and less costly than my method. And certainly less of a storage situation.
It usually ends up that the piece I bought the most of isn't used, and the piece I bought the least of, is the one I should have used as the dominant fabric.
So, if I was buying on speculation, and my budget could handle it - I would probably buy at least three yards of each one. You could always make pillow cases/covers out of the left overs. Or a coordinating wall hanging.
Also need to take into consideration that you may or may not be able to add to the collection at a later time. Some designs just show up for a very brief time.
And this didn't really answer your question, did it.