"Arithmancy looks terrible," said Harry, picking up a very
complicated-looking number chart.
"Oh no, it's wonderful!" said Hermione earnestly. "It's my favorite subject!" (PA6)
Arithmancy class is taught at Hogwarts by Professor Vector.
In Professor Vector’s class, Arithmancy students are expected to write essays and to be able to understand complicated number charts, which are part of their homework (PA12).
Hermione Granger appears to be the only Gryffindor in her year who attempted an O.W.L. in this subject (which is her favourite) (PA12, PA16).
Arithmancy is required for a career as a curse-breaker at Gringotts Wizarding Bank (OP29), so Bill Weasley would most likely have taken the subject for his O.W.L.s and N.E.W.T.s while at Hogwarts.
From the Web
Writing by J K Rowling on WizardingWorld (Pottermore): Hogwarts School Subjects
Lexicon essay: Arithmancy by Selena Gallagher
Harry Potter Wiki: Arithmancy (class)
Teach Beside Me: Harry Potter STEM: Arithmancy Math
Beyond Hogwarts: Foretelling the Future by the Numbers - An Introduction to Arithmancy
WizardingWorld.com (Pottermore) features: