Car enthusiasts will frequently look for rare models that were only released in limited numbers. A vehicle's availability plays a large role in its overall value with much rarer varieties experiencing a high increase in price. This applies to older cars as well since they become more and more rare with the passing of time. Finding out how many of a certain car exists requires some research but is useful information to have for dealers and collectors looking to add a vehicle to their stock.
Look up the make and model of the car in automotive magazines. If the car is considered a classic or a rarity, articles talking about the car will frequently state how many were initially made and how many are currently still in existence.
Contact the manufacturer of the car if possible. They should have a record of how many vehicles remain of a particular model.
Call a dealership that specialises in the car company that made the original vehicle. They might have access to stock counts and repair records that could provide at least a "ball-park" figure on how many carss of a certain type are still around.
Attend auto shows. You will have access to a lot of very knowledgeable car enthusiasts who both work in the industry and collect cars themselves. By asking around, you should be able to locate someone credible who can give you a reliable figure about the remaining number of a particular car make and model.
Online sites can sometimes provide current stock numbers of various cars but double-check their authenticity before taking the site's figures at face value.