In this verse we have what appears to be an inappropriate use of the adverb about (under the assumption that it means ‘approximately’). For most readers the number 476 is already quite specific, so the use of about seems strange, especially since it is preceded by the phrase “making in the whole”. In other places in the text, “making in the whole” is never used with the word about:
One could, I suppose, interpret “about four hundred and seventy-six years” in Mosiah 6:4 as meaning that the numbering is accurate down to the nearest year. In other words, it is not precisely 476 years to the day. Of course, we would normally assume the same lack of preciseness for the two other instances of “making in the whole”, yet they do not use about.
As an approximator, about is often used in dealing with a person’s age, as in the following two cases from Mormon’s own history:
More generally, if the period of time is relatively short, about can reasonably occur:
Usage elsewhere in the Book of Mormon shows that the adverb about can be used when counting people. In the following examples, the number is definitely specific:
In the first example, one could interpret the about as implying that although a specific count was made, it wasn’t checked for accuracy. But it seems doubtful that such an approximation could be intended in the second example.
Of course, in most instances the numbers are definitely general rather than specific, so about is perfectly acceptable:
Similar usage can be found in the King James Bible—for instance, in Joshua 7:4 (“so there went up thither of the people about three thousand men”) and Acts 2:41 (“and that same day there were added unto them about three thousand souls”). But there are two biblical passages where about occurs with a specific number:
Of course, one could argue that the number 12 may have acted like the English word dozen, given the common use of 12 in referring to the 12 tribes of Israel and the 12 apostles. But the number 36 is not a general number; in fact, 36 is used in only one other passage in the biblical text, yet there it refers to a general count of 36,000 beef cattle: “thirty and six thousand beeves” (or similarly) in Numbers 31:38, 44. Thus the example from Joshua 7:5 shows that in the King James text there is at least one example where about modifies a specific number.
In the Book of Mormon, the use of about, even when the number is quite specific, seems to be intentional rather than an error. The critical text will retain this usage, despite its unusualness for readers.
Summary: The Book of Mormon text allows about to modify specific numbers; such usage in Mosiah 6:4, Mosiah 18:16, and Ether 6:16 appears to be intended.