By default, Internet Explorer 9 shows 2 rows of your oft-visited sites on the about:tabs homepage, with each row having 5 thumbnails. However, if you wish, you can customize IE9 to make it show more rows with a simple registry tweak.
To do so, open regedit and navigate to the following key:
In the right pane in an empty spot right click and create a new DWORD and name it NumRows. Give it a value 3.
Restart Internet Explorer 9.
You will now see 3 rows being displayed.
You can give it any value between 2 and 5, with the default being 2.
Source: MSDN Blogs.
Lee adds: To use this registry edit, you will have to have history saved. If you have Delete Browsing History on Exit, checked, the registry entry for NewTabPage is deleted on exiting IE9 and you will have to recreate it each time you load IE9.