You can start in the "Free" version, but it is VERY limited
...and IF your tax return requires extra forms or is more than VERY simple..then you will have to upgrade to a Pay-for-it version to to file with TTX. I don't have thee exact figure...but perhaps ~half of the people who start with the Free TTX end up having to upgrade and pay for a higher version.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*