This one's fairly simple. There are a total of 12 vehicles you will have to steal. The missions are story-based such that new missions won't unlock until all current available missions (including car theft) are completed. Can't miss it.
Some additional thoughts:
* Many of these allow just enough time to complete the theft and drive back to whichever garage you're supposed to go to, making point-boosting low-yield (but not impossible) on some of these missions. The Shubert Beverly and the Military truck missions come to mind.
* Most car theft missions that end at the southeast (can't remember the name of the neighborhood) garage do allow ample time to blow up parked/passing cars to boost your demolition point multiplier.
* The following site has a pretty nice, brief walkthrough. I take no credit for it. http://www.brighthub.com/video-games/console/articles/88465....