I have a specific model file which I cannot add more than 8 materials to. I couldn't find any documentation about there being a material limit, and on other model files, I can make more than 8 materials, so I think this is a bug. MedievalVillage.model. Sorry for external link, I couldn't paste the entire file in this post.

Thanks for reporting! I was able to reproduce the issue, it'll be fixed in the upcoming version Asset Forge 2.1.0 (expected this month)