There are no specific numbers, it's more about when you start to experience performance issues. Even then the first thing to do is trash the CPD file. When that fails to help, then start thinking about creating a second project.
As a vague idea without many images I expect to see that happen around 4/5000 topics. Large numbers of images will affect that.
If you think you will be hit by this issue, then it might be better to start with multiple projects with each holding logical groups of topics. Restructuring later will involve rather more work.
See www.grainge.org for RoboHelp and Authoring tips
Yes, I have a project about 80 word pages but at least 50-60% are graphics.
How would you suggest doing the multiple projects and how would you tie them
together. Thanks, Gloria