Question

How many AMS 2020 subject classifications to list?

Answer 1

Following the link at the bottom of the post there is a “How to use the MSC” tab. It gives some guidelines how to pick one primary. It also says you can have serval secondary if that is relevant.

I sometimes have a paper with just a primary. Sometimes my papers have one or many secondaries.

Answer 2

I always thought it was one primary subject number, and up to two secondary subject numbers.

Answer 3

I have another question: Has anyone gotten any use out of the classification system? It's never done any good for me.

Answer 4

Look up some similar papers by more senior researchers that you cite and how many classes and which they have. That will give you a pretty good idea how things are done about this in your area of maths.