You're not doing anything wrong - this is a bug that has been reported several times already. There is a work-around, but it's not really what you want to hear at the moment.The "solution", for those that have iClone 5, is to make an empty project in iClone 5 and set the number of frames to a high number and save it. Copy this blank project file into your iClone 6 project folder and open it. Use this project as a base for new projects and you can then render more frames.
I think that this problem was introduced with the 6.01 patch and Reallusion have not mentioned when it will be corrected.