Q: Am I understanding this right? The GIRM says two candles are sufficient for Mass but for Eucharistic Adoration 4-6 are required?
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A: Yes. GIRM 117 says two suffice for Mass. The same pertains to a communion service at a church (Holy Communion and Worship of the Eucharist 19). Exposition in a monstrance requires 4 or 6 candles. Exposition in a ciborium requires two (85).
Perhaps the restraint at the Mass is another reminder that we gather for the eucharist first to give thanks, not to adore.