You will hit your doc stack limit when you complete all the documents in your stack. If you're on the Starter plan this will be 10 docs; Pro will be 30 docs; Business will be 200 docs.
You complete a document by:
Printing it
Downloading it
Syncing it back to Google Drive or Dropbox
Sharing it
Into a Team or any folder in Lumin that is not “My Documents”
When someone accepts your request to view or edit a document
TIP: A doc can only be completed once per month. If you have downloaded it once, you can download or share it again as many times as you like within your billing month.