Why did the ancient Jews use a candelabrum with 7 candles?

Wikipedia put it this way:

The menorah (Hebrew: מְנוֹרָה‬) is described in the Bible as the seven-lamp (six branches) ancient Hebrew lampstand made of pure gold and used in the portable sanctuary set up by Moses in the wilderness and later in the Temple in Jerusalem. Fresh olive oil of the purest quality was burned daily to light its lamps. The menorah has been a symbol of Judaism since ancient times and is the emblem on the coat of arms of the modern state of Israel.

The menorah symbolized the ideal of universal enlightenment. The idea that the Menorah symbolizes wisdom is noted in the Talmud, for example, in the following: "Rabbi Isaac said: He who desires to become wise should incline to the south [when praying]. The symbol [by which to remember this] is that… the Menorah was on the southern side [of the Temple]."

The seven lamps allude to the branches of human knowledge, represented by the six lamps inclined inwards towards, and symbolically guided by, the light of God represented by the central lamp. The menorah also symbolizes the creation in seven days, with the center light representing the Sabbath.

Hanukkah Menorah

The Menorah is also a symbol closely associated with the Jewish holiday of Hanukkah (also spelled Chanukah). According to the Talmud, after the Seleucid desecration of the Jewish Temple in Jerusalem, there was only enough sealed (and therefore not desecrated) consecrated olive oil left to fuel the eternal flame in the Temple for one day. Miraculously, the oil burned for eight days, which was enough time to make new pure oil.

The Talmud (Menahot 28b) states that it is prohibited to use a seven-lamp menorah outside of the Temple. The Hanukkah menorah therefore has eight main branches, plus the raised ninth lamp set apart as the shamash (servant) light which is used to kindle the other lights. This type of menorah is called a hanukiah in Modern Hebrew. - Menorah

Silver "Hanukkah menorah" with eight branches

Wikipedia has this to say about a seven candelabrum in Christianity:

The New Testament book of Revelation refers to seven golden lampstands, representing the seven churches of Asia to which the revelation was sent (Ephesus, Smyrna, Pergamos, Thyatira, Sardis, Philadelphia and Laodicea), with 'one like a Son of Man' in their midst.

According to Clement of Alexandria and Philo Judaeus, the seven lamps of the golden menorah represented the seven classical planets in this order: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.

It is also said to symbolize the burning bush as seen by Moses on Mount Horeb (Exodus 3).

Kevin Conner has noted of the original menorah, described in Exodus 25, that each of the six tributary branches coming out of the main shaft was decorated with three sets of "cups... shaped like almond blossoms... a bulb and a flower..." (Exodus 25:33, NASB). This would create three sets of three units on each branch, a total of nine units per branch. The main shaft, however, had four sets of blossoms, bulbs and flowers, making a total of twelve units on the shaft (Exodus 25:34). This would create a total of 66 units, which Conner claims is a picture of the Protestant canon of scripture (containing 66 books). Moreover, Conner notes that the total decorative units on the shaft and three branches equate to 39 (the number of Old Testament books within Protestant versions of the Bible); and the units on the remaining three branches come to 27 (the number of New Testament books).[28] Conner connects this to Bible passages that speak of God's word as a light or lamp (e.g. Psalms 119:105; Psalms 119:130; cf. Proverbs 6:23). - Menorah

Reverse of 1590 coin in honor of Urban VII with menorah and the legend SIC•LUCEAT•LUX•VESTRA (Let your light so shine - Matt. 5:16)

Popes have seven candles on the altar during a papal Mass.

The seventh candle seems to be the jurisdiction candle.

The number of candles "at a pontifical high Mass, celebrated by the ordinary, seven candles are lighted. The seventh candle should be somewhat higher than the others, and should be placed at the middle of the altar in line with the other six. For this reason the altar crucifix is moved forward a little. In Requiem Masses, and at other liturgical services. e.g. Vespers, the seventh candle is not used. If the bishop celebrate outside his diocese. or if he be the administrator, auxiliary, or coadjutor, the seventh candle is not lighted." - Why do Popes have seven candlesticks on the altar at Mass?

Popes have also lit a menorah for Catholic liturgical services.

There has also been much criticism of the Jewish menorah on the altar of the Neocatechumenal Way. Actually, the menorah is only there during special occasions such as Easter Vigil. Below are photos of our last two Popes lighting the Jewish menorah. - The Catholic Church And Her Jewish Roots

Pictures of Pope Benedict lighting the menorah in 2001 can be seen here.