Jacob 1:11

wherefore, the people were desirous to retain in remembrance his name. And whoso should reign in his stead, were called by the people Second Nephi, Third Nephi, &c. according to the reigns of the kings; and thus they were called by the people, let them be of whatever name they would.

So the people wanted to remember his name. Whoever became king after him was called Second Nephi, Third Nephi, and so on. That’s what the people called them, no matter what their real names were.

They wanted to honor his memory, so whoever became king after him was called Second Nephi by the people. Then came Third Nephi, and so on. It didn’t matter what their actual names were—the people called them by Nephi’s name.