Mass can be celebrated many times a day. The Divine Liturgy is ordinarily celebrated by one priest on one altar once per day. Of course there are extraordinary cases, but Mass itself ordinarily is celebrated multiple times a day on the same altar even by the same priest.
The participation of the people is important in the Divine Liturgy and is explicit in its theology. For example, the parts of the people during the Anaphora is important in making the bread and wine into the Eucharist. In Catholic theology, the priest saying the words of institution is all that is needed. The Great Amen that is part of the Mass has really no bearing to the "validity" of the Eucharist. The Eucharist is the work of the priest alone and not of the corporal body of Christ that is gathered.
The Divine Liturgy is not a free-form prayer service were people should be making their personal petitions. There is, ideally and as intended, no "quiet time" for an individual making their own personal petitions and prayers to God. That can happen anytime, anywhere, outside of the Liturgy. The Liturgy is where everyone comes together and become that one body, one mind. That is why it is important that people are listening to the same thing, saying the same thing, doing the same thing. In Mass, people always look for that "Sacred Silence" after they receive Communion and before the priest starts the post-Communion prayers.
The Divine Liturgy is only under the direction and approval of the Bishop. A priest with no antimension (signifying the approval of the bishop) cannot celebrate the Liturgy and have the Eucharist. Latin Rite theology does not say this. A priest can say Mass anytime he wants, anywhere he wants. A bishop's permission is only for the licety of the Mass, not necessarily its validity. So a priest can still say Mass privately without the permission of the bishop of the diocese he is in, even if he is visiting. But if he celebrates a public Mass, then he is subject to sanctions by the local bishop. But the Mass is still considered a Mass. In fact, a priest can schism from the Catholic Church and still say the Mass validly, such as the SSPX and even the sedevacantists.