I have looked around the net and other sources for material relating to the number of Excelsior Calss vessels built by Starfleet and the most that I could find in one listing was 30. I found this in Ships of the Star Fleet Vol. 1 Cruisers 2377-2378 by Chris Wallace. While not cannon by any means, it does seem a more reasonable number of vessels.
If anyone is interested here is the link for the .pdf file for reference. http://www.asdb.net/asdb/docs/sotsf/SOTSF1.pdf