The extreme rating keeps you alive, barely.
Any decent sleeping bag will also have another rating, often called “comfort”. This rating should be low enough to cover the lowest temperature you’re going to use it for, but only if you’re male and the bag’s size fits you well.
Women (on average) and people who are smaller will need an even lower temperature rating.
Also, the rating assumes you’re using a very well-insulated sleeping mat underneath, the bag is new, and completely dry.
So factor in some safety margin.

A bag with an extreme rating of 20 will definitely not be warm enough to sleep well at a temperature of 32.