When talking about the people who graduated from a school or university, one person is an alumnus, while two people are alumni. One person is NOT an alumni [sic]. Alumni is plural, alumnus is singular.
To further muddy the waters a bit, there are also the feminine versions of these words: Alumna is the singular and alumnae is the plural form. That said, it is not totally incorrect to refer to a woman as an alumnus and two or more women as alumni.
If you are referring to co-ed alumni, then refer to them as alumni and don’t worry about using the more convoluted alumni and alumnae or alumni/alumnae.