The Nurarihyon page mentions her having 100 mamono become her and her husband's harem, so 101+ could be possible if the guy's pelvis and groin can keep up with it.
I think both the Houri and Nurarihyon wich mention gigantic harem, are probably only making forced reference to the original myth, in the case of the Houri, they are angel of love, who value actual romance, it's hard to imagine them forming such a harem.
And for the Nurarihyon, the "night parade of one undred demon" would probably be an hyperbole, just a way to say there is a lot of monster around the man.
I'm personally of the opinion that all you need are arch incubus levels of stamina, a cloning spell, and a genuine love for everyone in your harem to be successful in all regards
I fully agree with Naranja46, by the way.
Is it possible to train your stamina indefinitely (as much as you feel like it) or is there a certain limit to any man ?
It depend on two things.
1) How long can each mamono tolerate going without sex.
2) How many women can you fuck in a single day.
Demonic Energy and Foreplay are your friends. One to make you last longer because it's stupid and needs a nerf, the other to make the job easier and more fun.
Eh, while I'm sure an Incubus may have the proper stamina and lust to keep up, I think the problem is that he still has only one dick. Meaning only one wife can mount him at any given time and then the others will be pretty envious, even if they literally just had gone five rounds.