How many IRs are there in the US?

An article in the D-Lib Magazine (http://dlib.org/dlib/september05/lynch/09lynch.html) gives a snapshot answer to this as of early 2005.
At that time the number was small with as many being planned as existed at that time.

Also from the conclusions:

"Research libraries have taken on a leadership role in both policy formulation (including the framing of the necessary campus-wide conversations) and operational deployment roles for institutional repositories at our research universities. (We did not explore funding questions, which will be crucial going forward.) Clearly, while an institutional repository is recognized as an institutional service, library leadership is generally unquestioned; what varies from university to university is the extent of active collaboration by other campus units. Institutional repositories represent a critically important new policy and operational role for research libraries, and one that renews their connection with the core academic processes of the university."