Search Analytics is a WordPress plugin that will keep history of the search terms used by your users and group them in a set of statistics including the number of posts resulted from that search term.
Search Analytics can be installed via the WordPress Automatic Plugin Install page in the admin panel. It can also be downloaded from this repository or from the Search Analytics Plugin Page and installed manually.
After the installation and activation is complete you should visit the plugin's settings page ( Settings -> MWT: Search Analytics ) to make sure it is properly configured for your needs.
Also, you can visit the plugin's Trello Board. This should give you an overview on my plans regarding this plugin.