Here Oliver Cowdery initially wrote the singular number in the original manuscript; then somewhat later he inserted the plural s with heavier ink flow. This change probably occurred when Oliver read back the text to Joseph Smith. The correction does not appear to be due to editing since either the singular or plural is possible here. Elsewhere in the text we have examples of both singular number and plural numbers in reference to the greatness of a number; none of the following cases show any textual variation between number and numbers:
singular number (7 times)
plural numbers (4 times)
For each case involving greatness, we allow the earliest textual sources to determine whether we have number or numbers. The critical text will accept the plural numbers in Alma 30:2.
Summary: Accept in Alma 30:2 the plural numbers, the corrected reading in the original manuscript; theoretically, either number or numbers is possible in this context.