Is there a limit to the number of Vault document versions?
There is no limit to the number of versions a document can have.
Leverage Lifecycle States to help indicate which versions are Obsolete, Draft, Approved, etc. and to update security on older versions. Use the Lifecycle Delete minor versions feature to clean up minor versions so that major versions can delineate big changes. But, those are all optional. A document can accumulate weekly versions to keep a full history visible.
Vault User Documentation: Document Versions