If you've reached the limit of Unlisted Publications, you will not be able to publish new Unlisted Publications to your account. Additionally, any document beyond the limit will no longer be viewable. There are a few options available to you though.
1. The first option would be to make some of your Unlisted Publications public by publishing them. See Making Unlisted Publications Public.
2. If you have any Unlisted Publications that are old or no longer needed, you can delete them from your account to create more space. See Deleting Publications.
3. If you need to keep more documents as Unlisted Documents, you would need to upgrade to one of the Unlisted Publication Tiers listed on the Account Services Page.
NOTE: The Unlisted storage package will renew monthly or annually, until cancelled.