Only at this point do modern readers begin to read Mormon’s masterwork. His introduction was lost, and the history covered by that beginning was replaced by the small plates. Even though they covered the same time period, they did so with a different purpose, and with different motivations behind what they wrote. Only beginning with Mosiah do we get to read the work that Mormon spent so much time and effort on to create for us.
The beginning of the book of Mosiah has been lost. It has been suggested that there are two lost chapters based on the numbering in the printer’s manuscript, but that numbering begins with the book of Omni and an erroneous numbering of Words of Mormon as Chapter II of Omni, with Mosiah becoming Chapter III. Oliver Cowdery created those numbers after the fact, and, therefore, that cannot tell us how many chapters we are missing.
The way that Mormon created chapters tells us that we are unlikely to know how many chapters might have been missing, but we can know what content was missing. The book of Mosiah is named for Mosiah1, who became king in the land of Zarahemla. It is probable that the book began with his seating as king. Whether or not the exodus from the city of Nephi and finding the people of Zarahemla would have been included in the book of Mosiah, or at the end of the book of Nephi (which is the only other named book that we know of), cannot be known. Speculation would suggest that it came at the end of the book of Nephi as Mormon typically begins named books with the seating of the ruler for whom the book is named. Presumably, that followed large plate conventions.
Because the beginning of the book of Mosiah was lost along with the rest of the content on the 116 pages, we do not have a book header for the book of Mosiah. It is the only book that Mormon edited which does not have a book header. The reason would be that it was lost, not that he did not include it. Mormon’s consistency clearly indicates that it was originally included.
Because what we have retained of the book of Mosiah begins with Benjamin, we have lost the entirety of what Mormon might have wanted to say about the reign of his father.