The current hard-coded limit in the code is 20 buttons (the max number of buttons must be kept reasonable to fit a viewer window on small 1024x768 monitors): the viewer will adapt to the actual number of buttons used in the side bar panel XUI definition file.
You can therefore edit the latter and add buttons (named "btn13" to "btn20"), then place it in your custom skin override folder (beware, the latter has been renamed "cvlv-skins" in v1.32.2.27: see
the change log).
The method for editing the side bar has already been covered in
this post (simply read "cvlv-skins/default/xui/en-us" instead of "skins/default/xui/en-us" for the override folder).