How many times do we ask for forgiveness during Mass?
There are three parts of the Mass that are explicitly penitential. The first is, obviously, the Penitential Rite (“Lord have mercy, Christ have mercy, Lord have mercy”). The second is the Our Father, in which we ask God to “forgive us our trespasses.” And the third explicit mention is immediately before Communion when we pray: “Lord, I am not worthy that you should enter under my roof.”
There are also less explicit penitential aspects to the Mass. The Scripture readings call us to repentance, and the homily might do so as well. Additionally, kneeling before the Eucharist is a sign of penitence and humility in the presence of Christ. And sometimes the prayers of the Mass specifically call us to repentance.