Americans, like me, say "math".
Brits say "maths".
Not sure how the rest of the Anglosphere does it (Canada, Australia, NZ, or South Africa).
Americans call the last letter of the alphabet "zee". But Canadians do it the British way and say "zed".
Never thought about it before, but "mathematics" ends with an "s". So the abbreviation should to.
I suppose that we Americans conceive of math as being like sand or water. Though plural it comes to you en masse like sand or water.