Answer:
There are 9 candles that are lit on Hanukkah. One is the helper candle ("shamash") used to light the other 8, which symbolize the 8 days of Hanukkah.
A story in the Talmud (500 years after the Maccabean War) tells of a legendary miracle. After the war, When the Jews returned to rededicate the Ancient Temple, they found that there was only enough oil to keep the eternal lamp burning for 1 day. It would take 8 days to make more of this special oil. So they lit the lamp, and it miraculously burned for 8 days, giving them plenty of time to make more oil to keep the lamp burning.
Although this is just a legend, in modern times, we light candles to symbolize the miracle of the oil lasting 8 days.