Desirability, not rarity is what one should be concerned with. Is a second generation Camaro without power steering or power brakes rare? Who cares! Is it desirable? No! I guess my point is don't get hung up on rarity. Look more for something that you like.
How many cars were optioned like yours? There isn't any way to know exactly since such records were not kept. However, you can look at the production figures, interpolate, and make an educated guess. It isn't very accurate since certain combinations of options were more/less likely to be ordered together, but it is the best that can be done.
Let's take a hypothetical example of an 1980 Z28 with T-tops.