How many transistors on an IC?
Mark does have a good answer, but I would like to add a few things.

Why not ask the company? The worst they can do is tell you no. You would actually be amazed at how much information I have been able to get from companies that I never thought I would be able to get.

As far as your question about why they don't include those numbers in the datasheets. I think the best way to answer this is to ask why would they want to include those numbers in the datasheet? Datasheets are specifically designed to give you all of the information you need to select and operate the chip, no more and no less. Now no datasheet is perfect so there always is areas that are lacking and other areas that are overboard, but in general they are aiming to be as concise as possible while still giving you everything you need. So, in the chip selection and operation process, why would a user ever need to know how many transistors are in a PIC?

The marketing process for an IC is very much different then the market process for a computer. With computers they are able to use the "it has more transistors so its better" argument. With ICs, generally the people using them don't fall for this.

Now, there was a time when the number of transistors in an IC was actually a marketing item to compete against others building similar products. However, in today's world this isn't really needed.