That's a tough question to answer. There are some nice patterns that can easily be done in two contrasting colors. In that case, you would need to worry about purchasing enough fabric yardage for the size you want to make. Most patterns specify how much fabric you need and many now give options for fat quarters.
If you want to make it scrappy, then you can always buy more if you run out. Don't worry, you will soon have a stash to choose from like the rest of us fabricoholics. :D