How do the plan limits work for the form and portal?

The limit does not apply to sub-forms (child forms). It only applies to standalone (parent) forms. You can add unlimited child forms.

Each Portal can have an unlimited amount of forms, child forms, and tables within it. Those added functionalities don't affect your usage quota. It will still be counted as one extension.

Each Interface can have an unlimited amount of forms, child forms, and tables within it. Those added functionalities don't affect your usage quota. It will still be counted as one extension.