What is the average number of patents per inventor at the USPTO? That
is, on average how many patents does an inventor receive?

This link could be of your use.

www.albany.edu/econ/Research/2004/rdprod.pdf

I found that http://www.uspto.gov/go/taf/reports.htm#by_invt would
provide you with most of the data however your question is not
definitive. Most Patents have multiple people associated with them and
data tends to relate to the first person listed in the patent. There
is a table that lists the most prolific individuals and this may be
pertaint starting point.If specific data average does not exist
suggest that you take a statistically valid number of patents and
analyse these in detail to provide an overall average. Someone with
expertise in stats may be able to offer you the correct approach.