Carry comfort and ease of deployment will be more important issues than the amount of ammo.
If you carry as much ammo as you would normally, you will be ok. If circumstances arise where you have to use your self defense ammo, replenishing your supply after the incident will be the least dramatic task you are handling.
For carry comfort, the Smart Carry works fine for me, but deployment is not the easiest seated in a car. It will, however, transition from sitting in the car to standing outside pumping gas without adjustment.
I've found that any IWB carry position has comfort problems with placement directly at your side on the hip the easiest. The holster MUST have an extension, body side, that covers the entire slide for me to be able to carry in that position for any length of time. Deployment is pretty easy but the movement will be obvious to someone at the driver's window doing a car jacking. Transition from from in car to out is not bad. Covering shirt or jacket arrangement needs to be checked.
The easiest to deploy would be any hand gun you have in easy reach with your right hand. My concern here is the car jacking situation. A prime location for a car jacking would be at the traffic light at the end of a expressway exit ramp. The driver is still in the making the adjustment from high speed driving to more awareness of their surroundings and will likely be tired from a long stretch at the wheel. With someone in your driver's window, using your left hand will be difficult. I place one of my LCP's, unholdered, in my center console tray. I get in the habit of resting my hand on it when the car is stopped while waiting for a light or traffic to move.
Deployment is quick and easy, comfort isn't an issue. Transition from inside to outside does require re-holstering and stowing in a pocket.