For the large menorah that was kept in the temple, it was 7 for the 7 days of the week. On our chanukkias that we use today, it is 8 for the 8 days the oil burned plus one in extra one to light the rest.
1; 7 Seven days of the week. 1 in the middle Sabbath 3 on each side (7 branches) 2; 8 There was a time in Jewish history something was enough to last (burn) for seven days but as a necessity it lasted for 8 days. (8 branches) 3; 9 believable by the power of God the eight day thing occurred (8 days plus 1 God)