Is there a source to find how many 1968 Barracudas were painted a certain color?

No, there is no surviving record that breaks down paint colors. But from experience it is pretty clear that the three shades of Turquoise are some of the least common colors.

Mopar only started using "artistic" color names in 1968. In 1967, the names were purely descriptive: Dark Turquoise and Light Turquoise. In 68, they became Surf Turquoise and Mist Turquoise. By 1969, they were more imaginative, adding Seafoam Turquoise as well as color names like Limelight, Sandpebble Beige, Black Velvet. And of course, in 1970 they introduced the "high impact" colors with wild names like Go Mango and Plum Crazy.