Here the printer’s manuscript has the plural numbers, whereas the 1830 edition has the singular number. In standard English we expect the singular. And elsewhere in the text, there are eight examples of in number. Five of the examples of in number involve an actual cardinal number:
(In all of these cases, the word about appears in front of the number, even when the number is precise. As explained under Mosiah 6:4, such usage is not an error.) In two cases, in number is used negatively with the word few:
And in one case we get in number occurring with many:
There are, on the other hand, no examples of in numbers per se, but there is an example of “in great numbers”:
Excluding the case here in 3 Nephi 7:11, there are 16 instances of the plural numbers in the earliest text where the singular number could be substituted without any fundamental difference in meaning (here we ignore any subject-verb disagreement resulting from the substitution in number). In this list, there are three cases where a predicate adjective is used to refer to numbers:
The example from Alma 58:15 is quite similar to the reading of the printer’s manuscript here in 3 Nephi 7:11:
Moreover, in 3 Nephi 1:29 the text reads “they had many children which did grow up and began to wax strong in years”, thus providing parallel support for the plural phraseology “strong in numbers” here in 3 Nephi 7:11.
There are only two cases where number and numbers have been mixed up in the transmission of the text, and both were errors by Oliver Cowdery:
The second example shows that Oliver could have accidentally added the plural s to an original number in 3 Nephi 7:11. The first example, on the other hand, provides evidence for the opposite change, from numbers to number, although not by the 1830 typesetter. There is no example of the 1830 typesetter (or the typesetter for any other edition) ever mixing up number and numbers.
Thus the evidence is somewhat contradictory. Internal evidence (usage elsewhere in the text, in Alma 58:15 and in 3 Nephi 1:29) supports the plural “strong in numbers” here in 3 Nephi 7:11 (the reading in 𝓟) while errors in transmission support the singular “strong in number” (the 1830 reading). But the number of transmission errors is minimal. And here in 3 Nephi 7:11, the singular “strong in number” is the expected reading, so it would not be surprising for the text to change from “strong in numbers” to “strong in number”. In this case, the critical text will accept the more systematic but unexpected reading, “strong in numbers”, especially since there is indirect evidence in the text for the plural expression “strong in numbers”.
Summary: Restore in 3 Nephi 7:11 the reading in 𝓟, “strong in numbers”; the phraseology “we were not strong according to our numbers” in Alma 58:15 supports the plural numbers in “they were not so strong in numbers”.