What is the number of licensed Registered Nurses in California and
what percent of them are represented by collective bargaining (union
representation)? $25.

Here are figures from 2001:

"California is leading the way in organization of registered nurses;
of the more than 130,000 active hospital RNs in California, about 41
percent are unionized. The independent California Nurses Association,
a Labor Party sponsor and UE ally, represents more registered nurses
than any other union in California. In April alone the CNA organized
more than 2,000 registered nurses at seven hospitals."

http://www.ranknfile-ue.org/uen_0601_rncourtdecision.html