Building a Rack
A rack is more than the actual protection though, so don't forget about draws (used to connect your placed protection to the rope), extra carabiners, and various other tools for self-rescue, rappelling, and building your own anchors. I like to carry about 10 complete draws, consisting of relatively short runners with non-locking carabiners on either end. In addition I'll bring at least three or four spare runners of variable length, slung across my torso, and a few spare 'biners to use them with. Having a couple extra locking carabiners is also useful, and you'll need something to hang all this gear from your harness as well.
Once you've made it to the top of your route you still need to build an anchor, so carrying a daisy chain for your own tie-in as well as a cord-o-let, a 6-8 foot 6mm cord (UIAA certified for climbing use) tied in a loop with a double fisherman's knot, is helpful. This will let you anchor yourself and your partner to a belay without using up valuable rope length and getting lost in a confusing tangle of knots.
Lastly, pick up a nut tool, a pointy device that will help you clean nuts and other protection out of tight spots in the rock.
After the Basics
Eventually you'll have collected the essentials and will be the proud owner of a working rack. But don't stop there. Over time you'll add to this collection, eventually having enough gear so you can choose items and be better prepared for the specific characteristics of the route you plan to climb. A nice selection of very small camming devices will help on those strenuous thin routes where hanging out to jimmy a Stopper placement isn't an option, and oversize cams will also help when off-width cracks find you one breath short of losing your grip.
Details mentioned in this article were accurate at the time of publication