Alfa Romeo Arna (1983-1987): only ~50,000 units. Some speculate that this car--a joint venture between Alfa Romeo and Nissan to create a budget car for the brand--is completely extinct today, with none still on the roads anywhere in Europe.
3rd-gen (2002-2004) Oldsmobile Bravada: 24,689 units.
Dodge Monaco (1990-1992): ~20,000 units for this placeholder model, and its sibling the Eagle Premier outsold it 4:1.
Saturn Astra (2008-2009): 18,256 units, with almost half (~9000) being sold in Illinois and Wisconsin, where Saturn dealer networks were strongest. The 3-door models were much rarer than the 5-door ones, though I cannot find sales figures.
Lancia Thesis (2001-2009): only ~16,000 units made in its 9-year production run.
Jaguar X-Type Sportwagon (2004-2009): only ~11,000 units, 1,587 of which were sold in the United States.
US-spec Alfa Romeo 164 (1989-1995): only ~3,500 units.
Cadillac ELR (2014-2016): only 2,956 units.
Pontiac Sunbird 2-door wagon (1978-1979): only 2,902 units.
Mazda Protege MazdaSpeed (only made in 2003): ~2,500 units.
Facelifted (made in 2004 only) Mitsubishi Diamante: only ~1,800 units.
Pontiac Solstice Coupe (2009-2010): 1,266 units.
Hybrid (made in 2008 only) Dodge Durango and Chrysler Aspen: only ~800 units, combined.
2nd-gen Kia K900 (2019-2020): only 790 units.
Citroen ZX 2.0 16V (1996-1998): only 558 units for the only exciting trim of the ZX.
Chevrolet Cobalt SS Turbocharged Sedan (made in 2009 only): 429 units.
Volvo V70 R-Design (made in 2010 only, US spec): only 388 units.
Rover 75 V8 (2005-2006): only 166 units for these Mustang-engined, rear-wheel drive (despite being based on a FWD platform) luxury saloons.
Coda EV (made in 2012 only): 117 units, all of which were sold in California.
Mitsubishi Dignity (2000-2001): only 59 units. The funny part was, Mitsubishi expected to sell 300 of them... per month, because its main rival, the Toyota Century, sold roughly 300 units per month and the Dignity was slightly less expensive.
Mitsubishi Pistachio (made in 1999 only): only 50 units; this was more or less a non-Kei version of the Minica, and only sold in Japan. All were sold to "environmentally conscious authorities and public corporations" and only had a 3-liter fuel tank.
2nd-gen Saab 9-5 SportKombi (made in 2011 only): a whopping 32 units.