The number of soldiers isn’t the issue. The two issues are the civilian casualties would be unacceptable for SK and the NK civilians would halt progress as they’d need support from the SK army.
The other one is that China would almost certainly invade from the north because NK is a buffer state between it and SK, a US ally.
America: where young people are coddled until they’re 18 then it’s either sell your body, sell your soul, or both multiple times over just to survive.