Here the original text read “and they were not in number a very few”, which Joseph Smith edited for the 1837 edition to read “and they were in number not a few”. He moved the not to after in number and deleted the word very. Joseph’s editing is the result of an attempt to eliminate the awkwardness of the original phraseology. Another possible emendation would have been to place in number after “not a (very) few”, as suggested by the only other example, a nearby one, where few occurs with in number:
Yet the occurrence of in number before “not a (very) few” is possible, especially since prepositional phrases are sometimes placed awkwardly in the Book of Mormon text; see, for instance, the discussion under Mosiah 26:23 and, more generally, under displaced prepositional phrases in volume 3.
The equivalent statement in Alma 17:37 (“yea and they were not few in number”) suggests various emendations for Alma 17:34, not only moving in number (discussed above) but also removing the a before few (as well as the very). Even here, there is independent evidence in the text for the a before few (but not for the very) in the phrase “not a (very) few”:
It should be noted that the first of these comes from an Isaiah quotation (Isaiah 10:7).
Despite the awkwardness of Alma 17:34 (“and they were not in number a very few”), the critical text will restore this reading since it appears to be fully intended.
Summary: Restore in Alma 17:34 the original reading: “and they were not in number a very few”; the meaning here is essentially the same as in Alma 17:37: “yea and they were not few in number”.