I need to convert 600 slides to HTML5, is there a limit on the number of slides that can be converted?
No, there isn't. You can do it , there isn't a limit.
I have checked the software's website and there was nothing regarding the limitation of files. You can convert as many files as you want (PowerPoint) to the HTML5 standard. However, to be able to do that, the full version of the software is required to be installed on your computer. A trial/demo mode won't allow you to perform the necessary task.
For more information, access the official website: http://www.digitalofficepro.com/powerpoint/powerpoint-to-html5-converter.html