What mean you exactly?
After highlighting four passages (sentences, paragraphs, etc.) the 'highlight' menu item is grayed out for a fifth passage. In addition, ctrl-m doesn't work either. I tested this on my system and so far I haven't found a limit to the number of highlights I can make. If the epub's DRM is marked 'social', is there a setting we haven't found that limits the number of highlights either of us can make?