The original manuscript is not extant for rejoicing(s), but the correction in the printer’s manuscript appears to be virtually immediate since there is no change in the level of ink flow. Elsewhere in the earliest text, there are examples of both the singular noun rejoicing and the plural rejoicings (six of the singular, three of the plural), including one other plural example in this chapter: “that we may have rejoicings in the joy of our brethren” (Alma 30:34). So either singular or plural is possible. Only one of these nine other examples involves variation in number: namely, in Helaman 3:31, where Oliver initially wrote rejoiceings in 𝓟 but then erased the plural s. Yet it is possible that the original text for Helaman 3:31 may have read in the plural (see the discussion under that passage).
Summary: Accept in Alma 30:22 the virtually immediate correction in the printer’s manuscript of the singular rejoicing to the plural rejoicings, the probable reading of the original manuscript (which is not extant here).