Depend of the mamono, depend of you.
For example, Devil Bug or Kherpi would be perfectly ok with a 20+ harem with their own kind.
However, Will o the Wisp or Lamia, would probably obliterate any woman trying to be intimate.
That's unknown. It may depend on the man's capacity and the types of Mamono in the harem. I think 20+ would probably be the biggest they normally get. That being said, the Houri page mentions a story of a single man being married to 72 Houris.
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.