Update
Airtable pricing plans have changed in September 2023. The extension limit does not exist anymore.
The 10 extensions limit in your Airtable plan only applies to extensions installed from the Airtable Marketplace. When you use miniExtensions, it connects through the new OAuth integration system. You can create an unlimited number of forms/portals (and other extensions) directly on our site without installing any extensions inside Airtable.
💡Note
Airtable has recently renamed the marketplace apps to extensions which might cause some confusion.