The menorah today has nine branches: one for each of the eight days of the Hanukkah miracle, and the central stem holding the shamash. The oldest known depictions of menorahs were different. They usually had six arms plus the central stem, and some rare ones have only four.A long article that surveys the uses and forms of the menorah through late antiquity, with a brief stop in the present.
There is no consensus on how today's nine-armed candelabrum used in commemorating Hannukkah emerged, especially given the divine directive to make menorahs with seven branches for other purposes entirely, but there are some intriguing ideas.
Hanukkah itself commemorates the purported miracle of one day's oil lasting eight days, at the height of the Maccabean revolution against Greek oppressors in the 2nd century B.C.E. Nowhere do Jewish sources dictate that a menorah be lit in commemoration, but that is the practice nowadays. As for menorahs themselves, one of the earliest-known depictions is from some 200 years later, and it's in Rome.
However, the oldest reference to a menorah is from a much earlier time, a time from which we have no archaeological evidence at all: the wandering of the Jews in the desert.
Many past posts on ancient menorahs are here and here and links.