Don't do that.
Off the top of my head, I'd guess that the average ignition lock cylinder replacement on your average automobile runs around $375 to $500. The more weight you have on your key chain, the more stress there is on the ignition key. And the more stress there is on your ignition key, the more stress there is on your ignition lock cylinder. Why make those things wear out any faster than they need to?
"When you're driving, all the weight that's on your key ring is is motion, and that motion is really hard on the lock cylinder," says my service department's head technician / electrical system guru. "When a car comes to me and the key ring has about ten pounds' worth of keys on it, I know what the lock cylinder will look like before I even have it out. The tumblers and other mechanisms will have taken a mile-by-mile beating."
While it might be inconvenient to keep as few keys as possible on your car-key ring, it could pay off in the long run. And not only in dollars.
Because when your ignition lock cylinder goes, you won't.