Mosiah 18:16 - 18:20

Mosiah 18:16 And after this manner he did baptize every one that went forth to the place of Mormon; and they were in number about two hundred and four souls; yea, and they were baptized in the waters of Mormon, and were filled with the grace of God.

In total, 204 individuals were baptized that day by Alma. He probably gave them the gift of the Holy Ghost as well.
[Where did Alma get this authority if all king Noah’s priest were corrupt? Either he was ordained by heavenly beings from heaven or it was somehow still in force by his ordination as a priest. It may that as Noah replaced the original priest with his own hand picked or at least some if not all of them, the authority was given to them by one who was worthy to do so. It maybe that they were ok at the beginning of their tenure, but became corrupt after a while. However on repentance, which Alma has just gone through, the power was returned to him.]

Mosiah 18:17 And they were called the church of God, or the church of Christ, from that time forward. And it came to pass that whosoever was baptized by the power and authority of God was added to his church.

Alma and his converts were now call members of the Church of Christ as were any others that join the group who were meeting in secret at the waters of Mormon.

Mosiah 18:18 And it came to pass that Alma, having authority from God, ordained priests; even one priest to every fifty of their number did he ordain to preach unto them, and to teach them concerning the things pertaining to the kingdom of God.

Alma having authority from God, organized the Church with priests over groups of 50. Alma then preached to the priest of 50 and they in turn preached to their 50 constituents.

Mosiah 18:19 And he commanded them that they should teach nothing save it were the things which he had taught, and which had been spoken by the mouth of the holy prophets.

These priests were commanded to only preach the gospel as he instructed them which he learned in the scriptures.

Mosiah 18:20 Yea, even he commanded them that they should preach nothing save it were repentance and faith on the Lord, who had redeemed his people.

Which is only the gospel of repentance and faith on the Lord which is the only path to follow to return to God.