"Ancient kings" is a very large umbrella term, covering millennia of history. I'd just note that we have to be very, very cautious when our sources mention wives and concubines. First of all because, as a rule, only politically important marriages would have been lavishly celebrated, marriages where there's an interest to both sides that this marriage become known.

Much more importantly, however, is that ancient sources have a very cavalier approach towards marriage and men's rights over women. You often read descriptions of the sort that "after the war, the men were killed and the women were taken captive." These are often the "wives" that show up later in large numbers in some harem. It's an euphemism, or, even more disturbingly, it's a way of referring to women who could for what matters be described as sex slaves. That's the kind of "wife" that's understood when you're dealing with Agamemnon's reference to Chryseis in the Iliad, to the women that Gengis Khan "took as wives" in the Secret History, or countless others.

There will be numberless nuances depending on the time period we're talking about, but I'll stop here.