How do I go about finding out about how many cars were built with the specs I have?

Kind of hard to say without knowing what "car you have". If its a Chevelle, there is no "list". A site member has compiled a poster showing the best estimate based on production figures. To take it down to individual options is strictly a guess.
http://www.designscomputed.com/auto_pub/a_body_68_72.html