For a regular regents diploma you only need 1 math regent – Algebra is enough. However, in order to get a regents diploma, you first have to qualify for a high school diploma which includes passing 3 years of math. (That’s the reason most schools teach Geometry and Algebra 2 as well.)
Check out http://www.emsc.nysed.gov/ciai/gradreq/GradReq3columnStyle7_1.pdf – this page is from the New York State.