There are numerous practical reasons for being sure you have more than enough wood to work with when building a project. Here are a few:
Arranging Grain Patterns and Color
Trees are a natural resource and they grow in strange ways. Therefore lumber can have all sorts of grain patterns and color changes (subtle or abrubt), so when you're working on a project you'll often want to arrange your parts in a way that's pleasing to you. That means you'll need enough to work with to get it just right.
Working Around Defects
If you prefer clear wood, we have a surprise for you. There is no grade of lumber that promises to be 100% clear 100% of the time on both sides of a board.
That means you'll have to anticipate working around parts of boards you don't like.
Making (and Fixing) Mistakes
If you've ever made a wood project before, you'll know how easy it is to make a mistake or cut a piece just a little too short.
It happens to all of us, even professionals. So plan on getting a little extra - because the only thing worse than buying too much wood is not buying enough wood.
Testing Wood Finishes or Stain Colors
It's best to have some pieces left over that are at least 6" long if you want to be able to test some stains or different finishes. The bigger the sample, the better you can judge the finish.
Tool Set Up and Joinery Tests
This is lesson many of us don't learn until we've made a few projects. A lot of times you need to make a few test cuts to dial in your set up for cutting joinery. To get the best results, it's best to do that with scraps that are the same thickness or the same width as your project parts.
The trick is how to buy enough without buying too much.
So expect to add on a waste factor of something between 1.5 and 2 (depending how picky you are about perfection).
That means figure out the board footage of your project parts, then multiply that by 1.5 to 2.
If your project parts equal 8.2 board feet, expect to buy at least 13 board feet.
9 would definitely not be enough. 20 would leave you a lot extra.