There is not really a great way to do this. The way I have seen posted about that works but is not perfect is to just divide your results by 30 days. It will not be an exactly right but it will be very close. The more months this formula calculates the more incorrect it will be. If you need to calculate months for a year or less I think it is a good solution.
This needs to be a formula numeric field. If you test this by September 1st 2020 - September 1st 2021 you get -12.16 for the months as 12 months have passed. You could probably clean up the number as well and not use decimals.
([date 1] - [date 2]) / days(30)
