If you have reached your plan's limit of Unlisted content, you will not be able to upload new Unlisted content to your account. Additionally, any document beyond the limit will no longer be viewable. There are, however, several options to create space for more Unlisted content in your account.
1. The first option would be to make some of your Unlisted content public by publishing them. See Making Unlisted Content Public.
2. If you have any Unlisted content that are old or no longer needed, you can delete them from your account to create more space. See Deleting Content.
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 content 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.