I don't know if there is a maximum. The reason being, although the vault view you see is layered like a windows folder structure, it is meerly an appearance created by WPDM. If you were to navigate to the vault folder using windows explorer, you would notice the file structure is relatively flat. Once you get into the projects folder you should notice that even the "subprojects" appear, not within their parent project. The perceived structure is actually controlled through metadata contained within each file that tells WPDM where its used, files it references, et al. I personally have seen vaults that have 6 or 7 layers of subprojects without any issues.
I don't know if there is a maximum. The reason being, although the vault view you see is layered like a windows folder structure, it is meerly an appearance created by WPDM. If you were to navigate to the vault folder using windows explorer, you would notice the file structure is relatively flat. Once you get into the projects folder you should notice that even the "subprojects" appear, not within their parent project. The perceived structure is actually controlled through metadata contained within each file that tells WPDM where its used, files it references, et al. I personally have seen vaults that have 6 or 7 layers of subprojects without any issues.
Hope this helps.