Correct, the XJ6C is comparatively rare, however, Jaguar produced 6-cylinder and 12-cylinder versions, in both Jaguar and Daimler (not available in the USA) configurations. In terms of rarity, the Daimler Double-six coupe is the tops, with only 407 being made, followed by the Daimler 4.2C with 1,677. The Jaguar XJ5.3C had 1,855. Then comes the more "common" XJ6C, with 6,487 produced.
Production figures courtesy of Nigel Thorley,s "Original Jaguar XJ".