The plural of alumna is alumnae.
A female graduate or former student of a school, college, or university.
The word alumna refers to a single female graduate and multiple female graduates are known as alumnae.
For the singular or plural of male or mixed graduates see 'alumnus or alumni?'.
The word alumna has Latin origin and its first known use in the English language dates only as far back as 1843.
Singular Example: One of Harvard's most noteable alumna in the world of entertainment is movie star Natalie Portman.
Plural Example: More noteable alumnae of Harvard include Michelle Obama and Youtube CEO Susan Wojcicki.
These days women dominate men in higher education, at a proportion of roughly 3:2 graduate students, according to studies. The field of education is especially high for rates of alumnae against their male counterparts.
Sources:
https://www.statista.com/statistics/184260/educational-attainment-in-the-us/
https://www.merriam-webster.com/dictionary/alumna#h1
https://www.businessinsider.com/most-famous-harvard-alumni-2013-10