What are the capabilities of the Free Account?
How many forms can be created and how many responses for the Free Account?
Is there ever a requirement to convert to a paid account?
The Free accounts don't change in any way.
The "Free" level account never requires an upgrade, one could continue to use "Free" indefinitally.
Free account allows one online form that can collect up to 50 responses at one time.
Here is a comparison of the features in the Free and both levels of paid subscriptions: https://www.acrobat.com/formscentral/en/features/compare-plans.html