I can’t comment authoritatively on the subject of archaeology, but I can provide some information on why those unlikely, massive armies might have been mentioned in the Old Testament. This issue crops up often in Biblical census numberings of Israel and Judah. For example, the number of Israelites in the exodus is reported as 600,000 adult men, not including their households [Exodus 12:37]. Only a few centuries later, King David’s census reported 1.3 million men of military age in Israel [2 Samuel 24:9]. These counts present a historical issue when the Encyclopedia Brittanica estimates that the total population of Egypt during the rough era of the Exodus was 2-3 million. There are several explanations.

One explanation comes from ancient Near-Eastern number theory, in which large, rounded numbers represented completeness. One example occurs in the book of Revelation, which depicts of the number of those saved by Christ as 144,000 exactly. This number probably wasn’t intended to be a prediction, but a representation of the number of the tribes of Israel squared, multiplied by a thousand for completeness. The total, in the eyes of an ancient Near-Eastern reader, would’ve been irrelevant in the face of the symbolic weight of that number. In this view, what the Bible intends to convey in Chronicles isn’t a count as much as a feeling, that of overwhelming Ethiopian superiority and an unlikely victory by an outnumbered Jewish force.

Another possible explanation comes from Boyd Seevers in his book Old Testament Warfare. He wrote that the great numbers of soldiers the Bible reports could actually result from a mistranslation of the Hebrew word ‘eleph’ used in these counts. It can be translated either as ‘thousands’ (the traditional manner) or as ‘clan,’ representing a family group smaller than one of the Twelve Tribes but larger than one immediate familial line. This translation works especially well when you consider that Israelite soldiers of the time were levies, not a standing army, and they both mustered and served in their family units, as the story of David and Goliath illustrates [1 Samuel 17].

In that case, we can reinterpret the numbers of troops in the Ethiopian army to be much smaller, actually consisting of the equivalent of 600 Israelite tribal units worth of soldiers. Seevers estimates the average size of this group as 10-11 soldiers. This is derived from Numbers 1:21, which passage describes a military force as either 46,500 strong, or as 46 clans comprising 500 men. From that conversion factor, he reaches a figure of about 11 men per clan. We can implement that figure for a grand total of around 6,600 Ethiopian soldiers.

Just for clarification, what translation are you using? In the NIV it simply states thousands upon thousands. That could be 1,000*1,000=1,000,000 or just several thousand