A classic Mini Cooper has two doors. The standard saloon was designed as a compact two-door, four-seat car, with the front seats folding or sliding forward to let passengers reach the rear bench.
This layout reflected Alec Issigonis’s space-saving engineering. The engine was mounted transversely at the front and drove the front wheels, leaving no long transmission tunnel through the cabin. Small 10-inch wheels and door storage bins also helped preserve passenger room inside the short body.
The answer refers to the classic saloon, not every Mini-derived body style. The Mini family also included two-door estates, vans, pickups, convertibles and other variants; later estate versions are sometimes described as three-door vehicles because their rear tailgate is counted as an additional opening. That body-style terminology can cause confusion, but the classic Mini Cooper saloon is conventionally a two-door car.