I'm not sure that it matters too much. I would say, "I (he, she, they) have(has) a Bachelors in Applied Mathematics". Additionally I might say, "The school offers a Bachelors of Applied Mathematics." But I think it's really up to you.
I also found this answer that was quite useful, but as I said I do not think there is a rule that determines which to use.
[degree level] of [broad category] in [specialty]
[degree level] is Associate, Bachelor, Master, or Doctor. That list is in increasing order of how many years of schooling each involves.
[broad category] is Arts, Science, Law, Business Administration, and occasionally Divinity. A Law degree is, well, about law. Business Administration is related to running a business. Science is a degree involving a physical science, like chemistry or physics. Divinity is a degree in theology or otherwise related to a profession like being a minister or a missionary, though many schools of theology today call their degrees Arts instead of Divinity. Art is pretty much anything else, like history, literature, philosophy, etc. Sometimes you see a degree in Fine Arts meaning music, theater, painting, or the like, to distinguish it from "ordinary" Arts. (Business Administration is mostly used in "Master of Business Administration": I don't recall ever hearing someone say they have an Associates in Business Administration, etc, though there may be colleges out there that give such degrees.)
[specialty] could be almost anything, whatever the college teaches.
So you could have degrees like, "Associate of Arts in Literature", "Master of Science in Physics", "Doctor of Fine Arts in Icelandic Poetry", etc. My degree is "Bachelor of Science in Business Computer Science". My ex-wife has "Master of Arts in History and Museum Management". And so on.