I found that at maximum 9 verbs are displayed there. Unfortunately I would like to have 12 verbs displayed there. They are all available from the context menu of an object in the game pane, but not in the "places and objects" panel.
How can one set the number of verbs displayed there, if at all possible.
Thanks in advance!

There are 9 verb buttons for each pane in the basic HTML; the javascript just applies different names to them and hides any spares when an object is selected.

If you need more, then you can duplicate some of them using javascript. The script would be something like:

while (verbButtonCount++ < 12) {
  $('#cmdPlacesObjects1').clone(true).attr('id', 'cmdPlacesObjects' + verbButtonCount).appendTo('#placesObjectsButtons');
  $('#cmdInventory1').clone(true).attr('id', 'cmdInventory' + verbButtonCount).appendTo('#verbButtons');
}

To include this in a Quest game, you'd go to the "UI Initialisation Script" (on the game's "Advanced Scripts" tab) and include the JS as a single line:

JS.eval("while (verbButtonCount++ < 12) {$('#cmdPlacesObjects1').clone(true).attr('id', 'cmdPlacesObjects' + verbButtonCount).appendTo('#placesObjectsButtons');$('#cmdInventory1').clone(true).attr('id', 'cmdInventory' + verbButtonCount).appendTo('#verbButtons');}")

Just off the top of my head, I think that should work.

Thank you very much! This script does exactly what I wanted. Good job!