Hi Sylvaing,
basically, it should be possible to use more than 10 slides:
In the file mod_extroslideshow.xml, copy one of the fields named slideXcont, where X is the number of the slide, and replace X by 11, 12, etc
In the file default.php in the tmpl directory, copy one of the lines that get the params from the slideXcont fields and adapt the X to the numbers you assigned, etc. 11, 12 etc