How to display number of posts in author pane?
it appears you need another module that will feed this info to author pane. This module is User stats.

Apparently though User stats has only a development release for D7 and reading through this issue, I would be reluctant to implement it on a production site. Will keep you updated if I find a better way to deal with this.

Edit: I've tested the 'post count' functionality of the 7.x-1.x-dev version of User stats and it seems to be working fine.