Question: Were there multiple copies of the plates of brass?

Answer: When Abinadi quoted the words of Isaiah, he did not have access to the original plates of brass. The plates of brass were in the custody of King Mosiah, who had received them at his coronation from his father, King Benjamin.

Since Abinadi recited the Isaiah text nearly verbatim, it is assumed that he had access to the words of Isaiah that were on the plates of brass. This suggests that copies of the sacred text, in whatever form, must have been available to Abinadi and others.

Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 140