How many evolutionary biologists are there working in the U.S.?

Wow..that's a big question.  Given the number of subdisciplines
evolutionary biology encompasses and the number of different jobs an
evolutionary biologist could hold without actually using "evolutionary
biologist" as a job title, I'm not sure you could ever nail down a
number.

Try this for starters ..

http://evonet.sdsc.edu/evoscisociety/what_is_evo_biology.htm

--Good luck 

-K~

Your question is going to require a bit of research and a lot of
expertise. You may consider making your bid for the answer more
appropriate to the question.