You should be able to pick that number out of the line with a regular expression capture. Regular expressions are a very good way to match lines in the logmon probe in general, and you can use parantheses to capture parts of the match into variables. When you configure the variable in the logmon watcher, you just specify which capture fills the variable (counting from the first set of parentheses starting at the left as 1). So something like this might work for a regex:
/ (\d+) File Rotation Needed/
You can save the variable as QoS too. I just did something very similar to this recently, and I think it has been working well. (I should check it again now that it has been running a little while.)