There are a few different places you can find the title number for your car. The first place to look is on the car's certificate of title. The title number will be printed in the same section as the make and model of the vehicle. You can also check your registration card. This is the same as a driver's license, but it's usually easier to carry around than the title itself.
If you can't find the title number, the next place to look is your state Department of Motor Vehicles (or a similar agency). The DMV has a database of all titles and other information about vehicles on file. In some cases, you can simply visit the DMV office and ask for the title number, or you can call them to get the information over the phone.
Another option is to contact your lienholder, which will have possession of the car's title until you pay off your loan or other debt associated with the vehicle. They can give you the title number or help you obtain a duplicate.
The title number isn't important in most situations, as it identifies the owner of a particular Certificate of Title. However, the DMV and other agencies use it for record-keeping purposes. You can also access other information about your car, such as the VIN, by looking at a copy of the car's registration or contacting your DMV or other government agency directly.