While the total number of 1968 Mustang Fastbacks with 390 S-code engines is documented (though still an estimate due to possible errors in reporting), the additional specifications (automatic transmission and dual quads) significantly narrow the possibilities and make it impossible to pinpoint an exact figure. The dual 4-barrel carburetor setup was a relatively rare option even within the already limited number of 390 S-code cars.
Therefore, the answer is: Unknown, but a very small number. We're talking likely dozens, not hundreds, of cars potentially fitting that description.
