the existence of Mr for men, Mrs / Ms for women and Mx for nonbinary implies the existence of at least 23 other genders: Ma, Mb, Mc, Md, Me, Mf, Mg, Mh, Mi, Mj, Mk, Ml, Mm, Mn, Mo, Mp, Mq, Mt, Mu, Mv, Mw, My, Mz
however, I think "Ma" can also be feminine (Ma Jenny is just short for Mother Jenny), and "Mq" is for a noble title (Marquess, like the Mq de Sade), and Mc is for Scots from Scotland and food from McDonald's. And My is just a possessive pronoun. So those probably aren't their own genders in the way that Mh clearly is.
So, we can conclude that roughly 22 total genders exist, but we can't be certain. For example, Mf clearly stands for Motherfucker, but that doesn't prove anything, since an estimated 16% of people would identify with and use the title Motherfucker. In addition, Mm can be a French title for Madame or Mademoiselle, but it is unclear whether this means Mm is just another female/feminine title or whether a French is a kind of gender. I will keep you all posted