What is the significance of the number of candles on the altar? Is there any significance with the number lit?
As with many of the visible elements of the liturgy, the presence of candles has long been both practical (as a source of light) and symbolic.
Some of the symbolism behind candles can be understood in the context of the Feast of the Presentation of the Lord (February 2), or what is sometimes called “Candlemas.” On this feast, commemorating when Mary and Joseph took Jesus to the Temple to dedicate Him to God along with Mary’s purification from having given birth, the Church traditionally blesses candles and commemorates the Presentation with a candle-light procession while recalling the words that Simeon spoke on this occasion of receiving Jesus, calling Jesus “a light to reveal [God] to the nations, and the glory of [His] people Israel.”
In addition, we can look to Easter for further symbolism of candles, and in particular to the Easter Vigil Mass on Holy Saturday night (after dark), when the Paschal Candle that symbolizes and proclaims Christ risen from the dead is blessed and lit from new fire. This Paschal Candle becomes the source of light for individual candles held by the people during the “Easter Proclamation,” or “Exsultet,” which is a hymn of praise to God for His saving works and particularly to Christ in His victory over death. Later in this Mass, the candles of the people are lit a second time as part of the renewal of baptismal promises, reminding us that we share in the light of the Risen Christ in our baptism.
Given these greater and more general symbols that are signified by the use of candles, the question of “how many are used at any given Mass” is based upon the occasion. It is notable, however, that the “occasion” as foreseen by the Extraordinary Form of the Mass is a matter of whether or not the Mass is a “Low Mass” (without singing) or a “High Mass” (with almost all the audible parts of the Mass being sung. For in a Low Mass, whether it be a weekday Mass or on the highest solemnities (such as Easter Sunday or Pentecost), there are only two candles on the altar. Meanwhile, if the Mass is a High Mass, even on a weekday, there are to be six candles burning.
Meanwhile in the Ordinary Form, the “occasion” is a matter of the level of solemnity of the given day in the liturgical calendar. For all Masses, there is always to be at least two candles lit. However, there can be some added solemnity when using more than two. Thus, the instruction says that on Sundays and Holy Days of obligation, there can be four or even six candles lit – as we do here. Finally, whenever the bishop offers Mass, the use of seven candles is invited.
Thus, candles indeed are an important symbol in themselves. That the number of candles lit can vary is simply an indication of the level of solemnity with which the Mass is offered – pointing to a higher solemnity of heaven itself.