The 1840 edition deleted the so before numerous; this is undoubtedly a typo. There are six other occurrences of “so numerous that” in the original text:
The so has never been omitted from any of these instances, although the that was removed from the example in Mosiah 22:2 (for discussion, see under that passage). To be sure, here in Alma 57:13 the secondary construction “our prisoners were numerous that …” is quite unacceptable as it stands; yet even then it was maintained in the RLDS textual tradition until the 1908 RLDS text restored the original so. The critical text will, of course, maintain the so in this phrase.
Summary: The 1840 edition’s loss of so in Alma 57:13 was clearly a typo; the critical text will maintain the original phraseology (“our prisoners were so numerous that …”).