In the US-CERT (United Stated Computer Emergency Readiness Team) year-end vulnerability summary, Windows had 812 vulnerabilities during the year. Another 2,058 vulnerabilities affected more than one operating system.
Although US-CERT didn't break out Mac vulnerabilities in a separate category, the Linux/Unix section listed more than 25 attributed to the Apple Computer operating system.
The end-of-year vulnerability score should be taken with a grain of salt, however, since US-CERT doesn't filter out updates (so one actual vulnerability can be counted numerous times) nor does it break out individual vulnerabilities from warnings that cover multiple bugs (as in the many Mac OS X vulnerability listings).
http://www.us-cert.gov/cas/bulletins/SB2005.html