You could crunch these numbers to come up with an estimate but it's a very simplistic method.....
7,199 Z/28s were produced in the '68 Model Year.
CRG estimates approximately 7% of '68 Camaros were Corvette Bronze.
40,977 of 235,147 cars were optioned with RS (equates to 17.43%).
So, if you assumed (no doubt incorrectly!) the selection of the RS option and body colour was applied evenly across all engine RPOs:
IF 7% of 7,199 Z/28s were Corvette Bronze, that's 504 cars; and IF 17.43% of those 504 cars had the RS package, then 88 cars had that combination. But again, this is a simplistic approach, and particularly on a color that wasn't available for the full production period, and an engine RPO that didn't apply to the convertible body style.
I'm sure the database keepers have a far more accurate assessment of the likely production numbers of that particular combination.