I see you submitted a support ticket for this and have received an answer there, but I wanted to post it here in case others are running into the same error.
The limit on this is 20 for a single user, in other words, one single user is able to authorize the same integration no more than 20 times.
If a user hits this error, they will need to revoke an authorization before being able to authorize a new one. They can do this by clicking their avatar > Integrations > Third-party integrations > Clicking on one of the appropriate integrations > Revoke access.
Thanks for highlighting that this knowledge isn't public-facing. We are working to get that information added to this article!