soccerchamp76, If I missed it, what kind of car. No one here has actually talked about the size of the space you are trying to provide sound for. If it were me (again depending on the size of the cabin, and where you are planning on placing the sub) I might drop down to a cleaner and better performing 8" Sub as opposed to one of the 10" that has been listed above.
Check out the
JL Audio 8W7
Diamond Audio CM3 08D2 or 08D4
All three of these subs will easily out perform any of the subs listed above.
As for ported vs sealed boxes. It totally depends on the speaker and where you are placing it. Just make sure you follow the manufacture's specs, they design their speakers to work with a specific amount to airflow and air volume.
As for your original question. If you purchase a high end 10" sub you should never need more than one, IF your intent is to provide quality sound INSIDE your car. For most cars, 2 high end 8" subs wired in series would even be better. If your intent is to disrupt the listening quality of the person stopped at the light next to you, or you want the world to know you have a loud system in your car, 8" speakers or a single 10" are not the way to go
