It used to be one, then two then now six.
Since there is no standard to follow car manufacturers will just include them in as they deem fit. Similar cars can be equiped with different amount of air bags.

A good example will be picanto. In Singapore, they are equiped with none. while in european nations they are equiped with two. Picanto is voted in Europe as one of the safest small cars.

Alot of experts argued about the fact of air bags on cars... being that it might be dangerous when fully inflated causing death in most severe situations. While others argued that seat beats are safe enough in most accidents. If you see the need for passenger side to be equiped with airbags why not asked how come the backseats are not??
