A taxicab is by definition a single automobile, so there is no need to suggest there is only one.
Larger vehicles that serve a similar function have different names like "bus", "trolley", "subway", "train", etc. These can have more than one car.
Trains and subways will sometimes call a "car" a "carriage", especially in BrE, but when used for single vehicles carriage refers to an older, horse-drawn vehicle.
(Edit) Please see the other answer for images commonly associated with carriage. Also, even though it's not part of the dictionary definition, it's possible that a carriage is a four-wheeled conveyance which does not move under its own power. For example, it is the locomotive which drives the train, and pulls (and/or pushes) a series of carriages. Similarly there still exist horse-drawn carriages, usually as a novelty.
The exception being the "electric carriage", pictured in the other answer, since replaced by the automobile.
(Side note) Like taxi, carriage implies a vehicle used to move people, not cargo. For a cargo-bearing vehicle pulled by draft animals, there is cart, wagon, or (in special cases) caravan, or (more archaic and obscure) dray, wain, or caisson.