Here the printer’s manuscript has an example of the correcting or, while the 1830 edition lacks it. Mormon added the or- phrase here because he wanted to differentiate between the entire army, which was under his overall command, and the ten thousand men under his immediate command in the battle. Verse 12 refers to Moroni’s ten thousand, and verses 13–14 list the names of the other Nephite leaders and the information that each has fallen along with “his ten thousand”. So Mormon’s correction helps the reader understand that Mormon is referring only to his own ten thousand here in verse 10.
It is very unlikely that the or was added here in Mormon 6:10; instead, it was probably in the original manuscript (and in the original text). There is considerable manuscript evidence for the occasional omission of the conjunction or; for three passages where or was initially omitted in the early text, see the discussion under Alma 1:30. And Alma 1:30 itself probably represents a fourth case where or was accidentally omitted in the early transmission of the text. In general, the tendency in the transmission of the text has been to omit small words rather than to add them.
There is one other example in the text of “yea or even”:
There are also a few examples of the corrective or immediately followed by even:
Thus the printer’s manuscript’s “yea or even” in Mormon 6:10 is acceptable and is probably the reading of the original manuscript.
Summary: Restore the corrective or in Mormon 6:10 (“yea or even my ten thousand”) since the construction “(yea) or even” is found elsewhere in the text; in addition, the tendency in the text is to omit short words such as or, which means that the shorter 1830 reading is probably the secondary one here.