@Phoenix of the Black Throne: You can count her if you want, but she wasn't born until after they were like "oh no there is only 198 of us!" and they she was taken to an alternate future. i understand she is at Utopia now but she wasn't really around.
so, i went and did some research to help with why there is more than 198. there is a group called Neo which apparently aren't exactly the same as mutants. Anteus, Elysian, jaeger, junction, and Salvo, are apart of this group. Ape-man i'm not exactly sure about i assume you mean Manikin and i don't know why he isn't one of the 198. Godfrey calthrope isn't on earth he is on some paradise world or something. Chimera is missing Clay i don't know about Crosta is with the x-men don't know why she isn't listed i don't know whats up with doctor nemesis. Doppleganger is a magic clone of spider-man so... unless there is another doppleganger i don't know about. Ernst and Cassandra Nova are the same person and apparently she isn't a mutant but an evil spirit twin of Professor X don't know whats up with Eden Fesi flying mutant i don't know about Milo gunderson is Cyber Hope Summers doesn't count. lois london i don't know about Proteus was half dead and half alive or something like that proton isn't a mutant Quentin Quire isn't Corporeal I'm not sure if Raptor is a mutant santa claus isn't a mutant (edit: i just noticed the question about the holiday special being canon, i don't think its canon but i could be wrong) I'm not sure about speed sunpyre is dead, the current one isn't really her or something amina synge is a mutate not a mutant threnody is dead or missing or something timeslip is depowered i'm not sure about topspin typhoid merry just wasn't on the list but was added later or something like that.
sorry if my post was kinda long and i hope this helps.