There are a few "services" or companies out there that do this (Chrysler definitely doesn't). Some of them are better than others, I have never used one of them as you can do most of the research yourself to a point. Mopar did not keep records on how many of a specific build was made, mainly because there are so many variables and at the time they didn't care. Depending on how far down the option list you want to do, almost everyone of them is almost a "one of one" or low number, but there isn't any way to prove that. What the companies do is maintain a registry of "known" cars, meaning cars that they have evidence of having existed or exists. From there, they take the actual production numbers (how many X model with Y engine and Z trans in T color were made), do some extrapolation and arrive at a probability of number built in a specific way. Its a little more complicated than this as there are some other indicators that they have like how many 6 way seat cars were built, how many stereo package cars, etc. (these are hypothetical).
Like I said, you can do a lot of this yourself, but you want a letter you are going to have to pay someone to do it. Also, research the company before paying them, they are not all the same.