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."
This candle seems to be now moved as "an astute Pray Tell reader writes in with an interesting detail on the altar candles:
I just managed to catch the end of the Pope’s Sunday Mass. Someone has moved the central candlestick (the one that says “I have jurisdiction here”) to the side, leaving what appears to be a much smaller-than-before crucifix alone in the middle, with three candles grouped in one corner, and four in the other . . ."
Abbots do not enjoy this privilege.
"The seven apocalyptic lamps (Revelation 1:12) might be in some way related to the menorah, but this would not have influenced their introduction into the liturgy.
The practice described in the seventh-century evidence was later discontinued in Rome although maintained in some monasteries. Sometime around the first half of the 11th century candles appear upon the altar itself. However, neither this practice nor the number of candles was universal.
It was not until the late 13th century that the custom of seven candles upon the altar for the bishop’s Mass was restored to the Roman liturgy. It was firmly established in a book of ceremonies written by Cardinal Giacomo Stefaneschi (1270-1343) who observed, “When the Pope celebrates solemnly, seven candles must be placed upon the altar.”
This practice continues today. For the extraordinary form the indications of the Ceremonial of Bishops for a bishop’s Pontifical Mass are to place seven candelabras upon the altar. The cross is placed in front of the middle high candle at the center.
The practice for the ordinary form is less precise. This is because it is permitted to have candles near the altar and not just upon it. Nos. 125 and 128 of the Ceremonial of Bishops foresee the possibility of having from two to seven acolytes with lighted candles form part of the entrance procession. These candles may then be located upon or near the altar.
Although it is not specifically stated in the liturgical book, and in accordance with the practice of papal celebrations, it is possible to have seven candles upon or near the altar before Mass begins. For the entrance procession two candle bearers accompany the processional cross and they may be followed by six torch bearers. In this case the candles used in the entrance procession are discreetly placed to one side once the procession is over. The two candles are used during the proclamation of the Gospel and at the end of Mass. The six torches accompany the thurible in front of the altar during the Eucharistic Prayer."
The Lamb's Supper
Revelation 1:20 also how seven candlesticks displayed at Mass could be seen as to demonstate the use of the pope's seventh candle (the jurisdiction candle):
The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches. - Revelation 1:20