Oliver explained that Mormon foresaw the approaching destruction and its parallel to the Jaredite destruction in the same place. Speaking from Mormon's perspective, and after describing the mile-wide valley west of the Hill Cumorah, Oliver wrote:
"In this vale lie commingled, in one mass of ruin the ashes of thousands, and in this vale was destined to consume the fair forms and vigerous systems of tens of thousands of the human race—blood mixed with blood, flesh with flesh, bones with bones and dust with dust!"
Oliver described the remains of the Jaredites as "the ashes of thousands." Not millions, but thousands. Not even tens of thousands. Just thousands.
When we read the Book of Mormon carefully, we recognize that Oliver was correct. The 8-day Jaredite battle at Cumorah could not have involved more than a few thousand, as we see from the count of the actual number killed on the last two days. Coriantumr realized that two million of his people had been killed long before they reached Ramah, or Cumorah. (Ether 13) There were additional battles leading up to Cumorah. Even after four years, they could gather only a relatively few people to Cumorah, so few that after six days of battle, there were only 121 people left. The next day, there were only 59 left. Even if we assume that half the people were killed each day, that calculates to about 7,744 on the first day of battle.
Hence, Oliver wrote that there were the "ashes of thousands," not even tens of thousands.
Same with the Nephites.
Oliver says "tens of thousands" were to be killed, including Lamanites and Nephites.
Mormon said he could see 20,000 from the top of Cumorah. (Mormon 6:11-12). The rest of his people, the ones Mormon lists in verses 13-15, had died long before the final battle at Cumorah. Mormon and Moroni could not see those dead people from Cumorah. Let's say an equivalent number of Lamanites were killed. That totals 40,000. This fits the "tens of thousands" Oliver mentioned.
You can read this right out of Joseph Smith's own history, titled History, 1834-1836, which is found in the Joseph Smith Papers here: http://www.josephsmithpapers.org/paper-summary/history-1834-1836/83. The portion I quoted is from this page: http://www.josephsmithpapers.org/paper-summary/history-1834-1836/92
Questions have been raised about my comments on the numbers of Nephites killed. Here are the verses from Mormon 6, with my commentary.
7 And it came to pass that my people, with their wives and their children,
[When Mormon wrote "my people" here, was he referring only to men? If he meant only men, why write "people" here? I think he was speaking about all the people, using the term "with" to mean "including." Other interpretations are also possible.]
did now behold the armies of the Lamanites marching towards them; and with that awful fear of death which fills the breasts of all the wicked, did they await to receive them.
8 And it came to pass that they came to battle against us, and every soul was filled with terror because of the greatness of their numbers. [This is a relative term, of course. There were lots of Lamanites, presumably more than there were Nephites, but Mormon gives not absolute or even estimated numbers.]
9 And it came to pass that they did fall upon my people with the sword, and with the bow, and with the arrow, and with the ax, and with all manner of weapons of war. [In terms of evidence, we would not expect to find metal or wood implements after 1400 years of exposure in western New York. Stone implements and components have been found in the vicinity of Cumorah, as well as on the hill itself.]
10 And it came to pass that my men were hewn down, yea, even my ten thousand who were with me,
[the phrase "yea, even" is used about 182 times in the Book of Mormon, usually to expand on or explain the previous thought. E.g., "becoming wicked, and wild, and ferocious, yea, even becoming Lamanites" "hope for a better world, yea, even a place at the right hand of God" "they came to a land, yea, even a very beautiful and pleasant land." Following this usage, I think Mormon is saying, "my men were hewn down; in fact, all ten thousand of my people were killed."]
and I fell wounded in the midst; and they passed by me that they did not put an end to my life.
11 And when they had gone through and hewn down all my people
[here, Mormon uses "my people" to refer to everyone, not just his men, unless we want to believe there were women and children in addition to the 24 survivors, which doesn't make sense.]
save it were twenty and four of us, (among whom was my son Moroni) and we having survived the dead of our people, did behold on the morrow, when the Lamanites had returned unto their camps, from the top of the hill Cumorah, the ten thousand of my people who were hewn down, being led in the front by me.
[There is an argument that the "ten thousand" refers to a military unit. But it can also refer to a group of people including men, women and children, which is how I think Mormon uses it here.]
12 And we also beheld the ten thousand of my people who were led by my son Moroni.
[Again, he doesn't specify men, but refers to "my people" who were led by Moroni. The other key point here is that from the top of Cumorah, Mormon could see his ten thousand and Moroni's ten thousand. But he doesn't say he can see anyone else.]
13 And behold, the ten thousand of Gidgiddonah had fallen, and he also in the midst.
[Notice the change. Instead of writing "we beheld" or "we did behold," Mormon writes "And behold." This phrase is used about 250 times in the Book of Mormon. It is used to call attention, not to recount what the writer is seeing. This is why the transition from verse 12 to 13 can be confusing if we're not reading carefully. The word "behold" can be a transitive verb meaning to observe. But is also used as an intransitive verb in the imperative to call attention to something. That's how Mormon uses it here. Notice, in verses 11 and 12 it's transitive because he writes "we beheld" and "we did behold," but in verse 13, it's intransitive. Here are other examples from the text. "he has testified aright unto us concerning our iniquities. And behold they are many." "And behold, there was peace in all the land." "And this is the commandment which I have received; and behold, they shall come forth."
The question is, why would Mormon write about his previous leaders and their people who had fallen?
First, he was with his son on top of the Hill Cumorah in New York looking back on everything that had happened. In verses 17-22, he reflects on the loss of his entire nation, the people who had refused his call to repent all the way back to the time when he was 15 and saw their wickedness and wanted to preach, but was prevented (Mormon 1:15). This was around 325 A.D., and in Chapter 6, he's writing around 385 A.D. He's looking back at 60 years of his life. In Chapter 5, he recounts how he agreed to lead the Nephite armies again in 379 A.D. He describes the conflict, the Lamanites burning towns, villages and cities, treading the Nephites under their feet, and sweeping down and destroying all the Nephites who were not fast enough to flee, even after the Nephites "did stand against them boldly." After 380, Mormon says he stopped writing about the "awful scene of blood and carnage" until he writes the letter in 384 and gathers "all the remainder of our people unto the land of Cumorah." It's possible they gathered more than 20,000 to the land of Cumorah, but by the time they retreated to the hill Cumorah, there were only the 20,000 left. That's why Oliver wrote that there were tens of thousands of bodies left, which presumably included dead Lamanites.]
14 And Lamah had fallen with his ten thousand; and Gilgal had fallen with his ten thousand; and Limhah had fallen with his ten thousand; and Jeneum had fallen with his ten thousand; and Cumenihah, and Moronihah, and Antionum, and Shiblom, and Shem, and Josh, had fallen with their ten thousand each.
[These were all people killed earlier in the Lamanites wars, or possibly killed in the land of Cumorah, but not at the hill (because Mormon couldn't see them from the top of the hill).]
15 And it came to pass that there were ten more who did fall by the sword, with their ten thousand each; yea, even all my people, save it were those twenty and four who were with me, and also a few who had escaped into the south countries, and a few who had deserted over unto the Lamanites, had fallen; and their flesh, and bones, and blood lay upon the face of the earth, being left by the hands of those who slew them to molder upon the land, and to crumble and to return to their mother earth.
[Over the course of years of fighting, it's not surprising that some would escape to the south and others would desert over to the Lamanites.]