The Mustang variant sold from 1984 to 1986 with fewer than 10,000 built was the Mustang SVO.
SVO stood for Special Vehicle Operations, Ford’s performance and motorsports division. The SVO was based on the Fox-platform Mustang but used a turbocharged 2.3-liter four-cylinder, a five-speed manual transmission, four-wheel disc brakes, distinctive 16-inch wheels, and unique aerodynamic bodywork. It was intended as a sharper, more European-style alternative to the V8 Mustang GT.
Production totaled 9,844 cars: 4,508 in 1984, 1,954 in 1985, and 3,382 in 1986. That total explains the question’s “fewer than 10,000” wording. A common mix-up is confusing the SVO with the 1984 G.T.350 anniversary model, which had 5,261 examples but was a separate appearance-and-equipment package.