Oliver Cowdery initially wrote “Ammon had made ready the chariots”; then almost immediately he supralinearly inserted horses & the, thus giving the expected conjoined construction “the horses and the chariots”, just as it appears earlier in this chapter and later in chapter 20 (except that the determiner here in Alma 18:12 is the rather than his):
Although this insertion in verse 12 could be due to editing, the virtual immediacy of the correction (there is no change in level of ink flow) suggests that 𝓞 had the complete conjoined construction, “the horses and the chariots”.
Summary: Maintain in Alma 18:12 the corrected reading in 𝓟, the conjoined expression “the horses and the chariots”.