I usually stick to one narration type per fic. Depending on what the story is, how many characters it deals with and what's going on.
For most fics, I use alternating first person (depending on who the scene is focused on or also to tell the reader something that that particular character would know that another would not) but for The Untold Origins of the Port Mafia, I used third-person omniscient because of the sheer number of characters and their complexity.
If the story I'm writing is one that's going to be a little out of character by nature (sickfics, relationship reveals, AUs) then the first person feels fine but, for fics where I feel staying true to the original character is very important, and I'm not sure how they would canonically express thoughts or feelings in the situations I want to put them in, I use third person.
When in first person, I can usually separate pov breaks with chapters depending on how long a scene is. So, chapters will vary in length, but if the pov is short enough, I'll put multiple in the same chapter because I don't love the idea of super short chapters. I do this to keep the story flowing, like how the camera in film and television changes focus on certain people at different times during a scene. I may do a poll about this later.
I've only ever stayed in one person's POV for the entire time in one fic. it was Chuuya for my fic More Than You Know, and even then, it's not done, so we might get some Dazai pov towards the end.
That brings us to another point. Dazai's character is so complex that if I'm trying to portray him in a close-to-canon way (as in character as possible), then I prefer the third-person for the risk of seeming OOC if I do first. IDK if that's just me or not. For my one-shot The One Day Return of Double Black, I entered with a third-person pov of Dazai and kept the rest as the first person with Chuuya. And I also did it for the first two chapters of my RANPOE!!! On ICE AU.