We have loaded in several complete car assemblies from major manufacturers. So yes, thousands of parts is possible. If you can’t share the file confidentially with us, then we can’t help much. In the 4.20 previews, you’ll have access to our 2-step importer process. You can load the assembly and verify it before deciding what to load from the assembly as geometry. This will allow you to figure out what is failing. Previews are a couple weeks away.