By "internal", I assume you mean within an organization? I am sure the stats would be dependent upon the size of the company, the focus of the newsletter, etc. Most companies would have no reason to release that info.
It would be interesting to know how and why e-newsletters are used and how successful the ROI is for Government organisations.
There seems to be stats available on the implementation and use of "Intranets". Obviously the Intranet is a more used tool, but the newsletter can contribute to the companies internal goals. Both of these tools are internal communication tools.
There is a lot of writing that says internal e-newsletters are popular (obviously they are.). There is also a lot of advice on what to include . There doesn't appear to be established stats to verify this...or investigate what types of topics are popular in an organisational internal newsletter.
For some companies a newsletter is an investment in time and staff, I require stats to qualify this investment.
A lot of this stuff would be organisation and newsletter specific, but I thought/hoped a study might have been completed.