Nephi's prophecy allows for a "few" more to see the plates. There were, in fact, 8 more.
"The following day, we returned, a cheerful, happy company. In a few days, we were followed by Joseph, Oliver and the Whitmers, who came to make us a visit, and make some arrangements about getting the book printed. fn Soon after they came, all the male part of the company, with my husband, Samuel and Hyrum, retired to a place where the family were in the habit of offering up their secret devotions to God. They went to this place, because it had been revealed to Joseph that the plates would be carried thither by one of the ancient Nephites. Here it was, that those eight witnesses, whose names are recorded in the Book of Mormon, looked upon them and handled them." (Smith, Lucy Mack. History Of Joseph Smith By His Mother Lucy Mack Smith, p. 154)
The footnote to this passage suggests the timing of the experience of the eight:
"It is probable, from this account, that the Eight Witnesses saw the plates during the week following the vision of the Three Witnesses. This would be the last week of June or the first week of July, 1829." (History Of Joseph Smith By His Mother Lucy Mack Smith, Footnotes)
Nephi explains this testimony of witnesses. Their word was to stand as a testimony for those who spoke from he dust. The Lord always intended that the Book of Mormon be a testimony of righteous people, not tangible objects. It is a book for faith.