Have never seen any specific numbers by color. (The data is obviously within the PHS records but it may still be in 107,112 individual records.) About the best you might do is an interpolation/guesstimate.
As a start, "The Definitive Firebird & Trans Am Guide 1967-1969" by Rocky Rotella shows 458 Sprint 6 Standard Converts and 199 Sprint 6 Deluxe Converts were produced in '68.
From there you might do a rough guess of the percent of Cameo Ivory '68s out of the 20 available colors (4 being rare special colors). For example, if you assume 5% of all '68s were that color; 458 standard Sprint converts times 5% = a very rough SWAG of 23