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.
Find your plan (you can choose the current one as well as upgrade or downgrade) and select your desired amount of Unlisted publications in the dropdown menu.
In order to be able to upload up to 100 Unlisted you would pay $5/month etc. Once you reach the limit you can always delete some of your Unlisted or upgrade again.
NOTE: The Unlisted storage package will renew monthly or annually, until cancelled.