The 1841 British edition omitted the expected the here in Mormon 6:12. Clearly, the text requires the the since Mormon referred earlier in verse 10 to his own ten thousand. Also note the use of the the in the following verse: “and behold the ten thousand of Gidgiddonah had fallen” (Mormon 6:13). The 1849 LDS edition restored the the here in Mormon 6:12.
Summary: Maintain in Mormon 6:12 the definite article the in “we also beheld the ten thousand of my people”, the earliest (and obviously correct) reading.