Open Defects count per month (RRDI report)

In the Request Metrics section there are Request Creation Metrics under Change Mangement section of the DW package.  One of the tutorials on this youtube channel shows how to use:

http://www.youtube.com/user/IBMJazz/videos?query=reporting

Of course, when you say "open" you _might_ mean what defects are not yet resolved, rather than simply created, but using the Creation metrics in conjunction with Request Status you can get historic information.