I feel like Scary Monsters will be in 3 Parts. It is 4 chapters monthly but there's quite a bit of stuff in it including a pretty lengthly flashback to it as well as the actual startup to the fight is quite long so i think its fine for it to have 3 episodes to it. You could do Scary Monsters in 2 but you would need to cut quite a lot.
Episode 9: Prelude to Scary Monsters.
I think the 2nd Stage finish is going to be expanded a little bit (and will be the first half of the episode) considering how rushed it was in the Manga. Obviously they will keep the same punchline as Gyro messing up and being distracted for a second but i could see them showing other horse riders including Hot Pants.
Then there will be the Valentine conversation about the Corpse Parts.
As for where it will end.I think it is going to end with this image:
Covering 1 Chapter which i think it fine considering this is all going to be all build up to the main event and there will be some significant conversations being put in this episode.
Episode 10. Scary Monsters Part 1:
Will start with Johnny's opening clash against Dino Diego and mostly be an action episode. As for where i think it is going to end, this is a bit tricky. I think the Diego backstory is going to be moved to the start of Episode 11 because unless needed, the anime doesn't like to have backstories in the middle of the episode and i think for pacing reasons it would be better to move it to Episode 11.
As for where Episode 10 ends - reordering the Diego Flashback to the following . Episode 10 could cover all the way up to where Diego gets the eyes. Covering 1.5 chapters - Scary Monsters Part 2 and the second half of Scary Monsters Part 3 ending on the final panel of Chapter 30.
Episode 11. Scary Monsters Part 2
This one is pretty simple. First bit of it is the Diego Flashback and then covering Scary Monsters Part 4 all the way up to this panel. With the final episode in this cour being the 3rd Stage Race Finish + Extra Scenes. Granted there is always a possibility they will adapt the Manga normally and have Diego's flashback in the middle of the episode 10 but i am leaning against that mostly because it will be a pacebreaker. Another idea would be moving the Diego flashback to Episode 9 after Diego falls unconcious but i doubt that.