Chicks need plenty of room to run and play, and they grow very fast, so 18" will be too small for that many chicks, especially if you plan on keeping them in the same brooder for the full six weeks. We use a large rubber maid container(shown above), it works great for up to 10 chicks, for 4 to 6 weeks. We keep it in our master bath tub, to keep the dogs and cats from bothering the chicks. You'll definitely need a brooder light, chicks need to be kept at a temp of 90 to 95 degrees, then reducing 5 degrees each week until they're fully feathered.
Make sure you have the coop ready before you bring them home. We made that mistake last year, thinking six weeks would be plenty of time, it wasn't. It was really stressful rushing to get it done while having babies, that were getting too big for their brooder.
If you haven't raised chickens before, it might be a good idea to get a how to book, on the subject. It's not hard, but there is a lot that has to be considered. My favorite book is Chick Days by Jenna Woginrich, it give new comers all the basic info they need, and has plenty of full color photos to illustrate all the how to's. Or, if you don't want to buy a book, you may be able to find
one at your local library. Research is the key to healthy happy chicks, and happy owners too....good luck!