Ammoron is the name of the brother and successor of Amalickiah. The name occurs 24 times in the text (from Alma 52:3 through Helaman 1:16), of which 16 are fully extant in 𝓞, and 15 of those read Ammoron, including the first occurrence (in Alma 52:3). 𝓟 consistently has the correct spelling Ammoron.
Here in Alma 52:12 the name Ammoron was accidentally replaced by Ammaron in the 1879 LDS edition, a simple typo. Everywhere else the 1879 edition maintained the correct Ammoron. This particular misspelling here in Alma 52:12 continued in a number of LDS editions that were set from the 1879 edition (namely, the 1888, 1902, 1906, and 1907 editions). The 1905 edition restored the correct Ammoron to the LDS text.
Elsewhere the Book of Mormon has three other names that are very similar to Ammoron, namely, Amaron (twice, in Omni 1:3–4), Ammaron (ten times, from 4 Nephi 1:47 through Mormon 4:23), and Amoron (once, in Moroni 9:7). There is a possibility that Ammaron could actually be Ammoron; for discussion, see under 4 Nephi 1:47. But there is no doubt here in the books of Alma and Helaman that the name for the brother of Amalickiah is Ammoron.
Summary: The correct spelling for the name of the brother of Amalickiah is Ammoron; 15 out of the 16 fully extant occurrences of the name in 𝓞, including the first occurrence, support this spelling.