No, the 10 extensions limit in your Airtable plan only applies to extensions installed from the Airtable marketplace. When you use miniExtensions, you have the option to install our 1 extension from the marketplace, but it's not mandatory. You can create an unlimited number of forms/portals (and other extensions) on our site without installing any extensions inside Airtable. We have no limits on our end.
Airtable has recently renamed the marketplace apps to extensions which might cause some confusion.