Conversion by the thousands seems incredible to modern day missionaries. But the Lord is capable of greater things than we can comprehend. A few instances in which thousands were gathered at once are given:
1) On the day of Pentecost, those who had heard the word were invited to repent and be baptized, and the same day there were added unto them about three thousand souls (Acts 2:41)
2) The success of the early brethren in Great Britain was astounding. By April of 1838 Heber C. Kimball had converted 1500:
"'The other brethren also met with remarkable success. The field was ripe, ready for the harvest, and thousands of the House of Israel were soon gathered into the fold.' By 1841, Elder Kimball with other members of the Quorum of the Twelve reported seven or eight or ten thousand persons being baptized. 'Of the converts made by the eight members of the Twelve during the years 1840 to 1841 few apostatized.' During the years 1840 to 1846, British converts sailed from England bound for Nauvoo. 'This was the beginning of a migration movement of such magnitude that the Church in America became predominantly English for the next fifty years.'" (Regional Studies in LDS History: British Isles, p. 107)
3) George Q. Cannon reaped a similar harvest on his mission to Hawaii:
Heber J. Grant
"During the few years he (George Q. Cannon) was there he translated the Book of Mormon into the Hawaiian language, and he baptized over 3,000 natives. I have never been among a people that seemed to me to be more sincere, to have a greater love for each other and for the Gospel, and a greater reverence for those who hold places of honor in the Church of Christ than I met on our recent visit to Hawaii." (Conference Report, Oct. 1935, p. 7)