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.