I wonder why a car has 4 wheels but has only one spare wheel. Is it that only one tyre can puncture at a time? If not, what if more than one tyre punctures on the go?
Many modern cars have no spare wheel at all. BMW have recently started implementing run-flat tyres across the range. Tyres which can, even with a puncture and no air pressure, be used at a reduced road speed.
Some other manufacturers have begun to include a compressed can of "tyre weld" or similar which attaches to the valve of a flat tyre and injects a sealant foam, again so the vehicle can be driven at a reduced road speed to a tyre bay for proper repair.
Carrying a spare wheel is quite an expensive business. It's expensive from a perspective of space, it's expensive from a perspective of weight and its expensive from the added production costs. However, generally a single spare is seen as a good compromise. There is nothing to stop a multiwheeled vehicle picking up more than one puncture at a time but this is an unlikely scenario.
Continent crossing vehicles (such as Dakar rally cars) will generally carry multiple spare wheels strapped to bonnets, roof bars or on the tailgate. This is because they may have to travel hundreds of miles before finding a garage.
For most consumer vehicles, they're usually never more than a few miles from a town or village with a tyre bay. Also, they're usually used on roads where busses operate and mobile phone coverage is good. If you were unlucky enough to have two punctures, the likelyhood is you can call a local garage, continue your journey on the bus or at worst case, have to flag down another passing car.
I should add that the most common reason why a four wheeled vehicle requires two tyre repairs is that a puncture has happened and the owner has found that the spare is also flat.
Take a look at some classic antiques. Certain models have two spares, one mounted in each of the front quarter panels behind the axle.
I found approximately a zillion examples here
As tires, and roadbeds, got better designed, the probability of getting a flat (or more) rapidly moved asymptotically close to zero. Savings in cost and weight are critical to sales success, hence the disappearance of spares.
With modern tires on decent-quality roads, you can expect a flat tire about once every 150,000 km. So the chance of having a flat within one given trip is very small (let's say 1/1000 if your trips are 150 km long on average). The chance of having two flat tires at the same time independently of each other becomes 1/1,000,000.
That chance is so small, the cost of carrying a second spare with its drawbacks (weight ~25 kg, increased fuel consumption, and the space it takes up) far outweighs the benefits.
If you have two flats simultaneously, chances are you'll have 4 flats because you've driven over agressive one-way traffic enforcement measures or a police blockade. 2 spares aren't going to help you in those situations.