We aren't using our imaginations here. We are assuming these Kings were serial monogamists, on a 24 hour changeover, with individual girls taken nightly or daily to the bedchamber. He probably had many large group sessions moving between the girls at his whim. The girls probably wacked themselves off watching the king at play amongst them. Hence they would have been getting more sex than simple maths suggests.
But still, you are right, still not getting enough. Oh man. I'll bet there were some terrible fates for those horny court blokes caught knocking of the concubines who must have been gagging for it.