Large groups of people converted at the same time

Scripture references

Acts 2:41 (ASV 1901)

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 1901)

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 spirit of God broke during the prayer meeting of believers in Shuntehfu. Many confessed their sins and 500 people acknowledge Christ as their Savior for the first time.
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.