Please, I want to know from Scriptures about mass prayer in New Testament worship. Can we pray together, when we come together, but each one prays his or her own prayer?
Worship as a church is distinct from individual worship. In a church service, everything is done to edify those gathered. "What is the outcome then, brethren? When you assemble, each one has a psalm, has a teaching, has a revelation, has a tongue, has an interpretation. Let all things be done for edification" (I Corinthians 14:26).
If everyone is individually praying, each is uplifting himself but no one else is being encouraged. "Let us hold fast the confession of our hope without wavering, for He who promised is faithful; and let us consider how to stimulate one another to love and good deeds, not forsaking our own assembling together, as is the habit of some, but encouraging one another; and all the more as you see the day drawing near" (Hebrews 10:23-25). Without that mutual edification, there would be no need to gather as a church. After all, prayers are offered all the time by Christians around the world in mass, but this is not the worship of the church.
In scolding the Corinthians for putting too much emphasis on speaking in other languages, Paul points out, "Otherwise if you bless in the spirit only, how will the one who fills the place of the ungifted say the "Amen" at your giving of thanks, since he does not know what you are saying? For you are giving thanks well enough, but the other person is not edified" (I Corinthians 14:16-17). Praying in a language that others cannot understand prevents the edification of those listening to the prayer. The same can be said if everyone is praying individual prayers at the same time. Others cannot agree to something they did not hear because they were too busy with their own prayer.
Assuming all are speaking their prayers out loud at the same time, the result would be a confusing babble. Another rule for worship is that people speak one at a time in addressing the assembly so that the assembly can understand what is being said. "If anyone speaks in a tongue, it should be by two or at the most three, and each in turn, and one must interpret; but if there is no interpreter, he must keep silent in the church; and let him speak to himself and to God. Let two or three prophets speak, and let the others pass judgment. But if a revelation is made to another who is seated, the first one must keep silent. For you can all prophesy one by one, so that all may learn and all may be exhorted" (I Corinthians 14:27-31).
Say your personal prayers on your own time, but in the worship of the saints, let all be lead to pray together.