In this passage the 1874 RLDS edition changed the plural flocks to the singular flock, probably accidentally since for virtually all other references to flocks in this narrative in Alma 17–19 regarding king Lamoni’s sheep, we have the plural flocks in the earliest text (29 out of 30 cases). The only exception is in Alma 17:27, where 𝓟 reads flock but the original text probably read flocks. See the discussion under that passage. Here in Alma 18:3, the RLDS text has continued with the singular flock, but the critical text will maintain the plural flocks, the earliest reading.
Summary: Maintain the plural flocks in Alma 18:3, the reading of the earliest textual sources.