Technically, there's no upper limit to the number of airbags you could place in a vehicle. A car could be designed with every interior surface ready to inflate during impact; however, that vehicle would be very expensive and very heavy. Cost and weight -- those are the two main limits to the use of airbags.
The installation of airbags costs money for both the materials and labor. Replacing an airbag that has already been deployed can cost anywhere from $500 to several thousand dollars. They cost significantly less when they're initially installed at the factory, but the cost adds up quickly as more and more airbags are added. For example, the side airbag option on a 2006 Ford Fusion adds almost $600 to the final cost of the car [source: CBS News].
Extra weight is another problem. Increased weight reduces fuel mileage. The U.S. Environmental Protection Agency estimates that each 100 lbs. of weight added to a car reduces mileage by two percent [source: Fueleconomy.gov]. While automakers don't typically release information about the weight of individual components, and an individual airbag doesn't weigh very much, multiple airbags can significantly increase overall vehicle weight. If you consider the airbag itself, the chemical propellants needed to inflate it, the wiring and sensors, plus the enclosures for all the components, you can see that filling a car with a bunch of airbags would hurt your fuel economy.
Sure, airbags may make your car slower and heavier, but there some definite advantages. We'll explore those on the next page.