Large groups of people converted at the same time

Scripture references

Acts 2:41 (ASV)

41They then that received his word were baptized: and there were added unto them in that day about three thousand souls.

Acts 4:4 (ASV)

4But many of them that heard the word believed; and the number of the men came to be about five thousand.

Stories

After the first Yagba believers endured persecution, a great revival occurred in which thousands were saved.
God answered the prayers of George Muller for his orphans. Large groups of them were converted several times, showing signs of true conversion such as concern for the soul of others and a hunger for prayer.
Bobby, the first Motilone to trust in Jesus, led his entire tribe to saving faith in one day.
Bruce Olson tried to tell a Yuko woman about Jesus. Later Jesus appeared to her in a vision with the result that she and nearly her whole village accepted Jesus.
The believers and missionaries at Shuntehfu were so burdened with prayer for their community, that they would rather pray than eat. As a result, the spirit of God came with such power during the meetings, that five hundred people acknowledged Christ as their Savior for the first time.
At Pingtuchow, Jonathan Goforth faced every imaginable obstacle that could hinder the work of the Holy Spirit. There were wide disagreements amongst the missionaries, including disunity over non-essential doctrines. However, things changed on the sixth day when the leading missionary was convicted and apologized to his opponent. That began a reconciliation chain culminating in the transformation of that church; everyone confessed their sin. Three years later, more than three thousand have been added to the church in that region.
George Müller and his helpers fervently prayed for a visitation of grace upon the orphans. After Mr. Müller was already in America for a speaking tour he received word that these prayers had been answered, resulting in the conversion of five hundred and twelve orphans.
A seventeen-year-old orphan girl, well-known for her hardness of heart, was converted on her sick bed. Her sudden and complete transformation was such a powerful witness to the other orphans, that it led to the most extensive spiritual awakening up to that time, with hundreds of children being converted.
When Jonathan Goforth and his wife, Rosalind were invited to hold meetings at a certain place, she almost did not go. But she did anyway, and later rejoiced, for the Lord brought thousands of souls into the Kingdom through those meetings.
Tens of thousands of the Dani tribe came to Christ in a short period of time, something the missionaries were totally unprepared for.
Robert Gribble was moved to change his residence, he obeyed and was led to a village that had a great need for the gospel. He reached the unreached with the Gospel of Christ, by the end of his seven years' stay, he had introduced the gospel to five different places and between eighty to hundred people had converted.
When Captain Mitsuo Fuchida; pearl harbor's commander, gave the powerful testimony of his conversion to Christianity and the power of forgiveness over revenge to a crowd in Osaka, he appealed to the crowd's war weariness, declaring that "real peace comes only through Christ" - about 500 people accepted the call to come Jesus that day.
In a dream, Yun meets a young man who invites him to come share the gospel in his village. The next day Yun found the exact man he saw in his dream was waiting for him and spoke the same message as in the dream.
Titus Coan depended only on God's Spirit to bring home the words whenever he preached. And the Gospel he declared wrought changes in the hearts and lives of those who listened; sins were confessed, quarrels reconciled, idolatry renounced and God's cause; greatly advanced.