It's hard to give an exact number, but I'd say Itadori has eaten at least 10 Sukuna fingers in the manga. The number keeps fluctuating based on the plot and his encounters with various challenges.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Perhaps it was because he did not pay attention to the story and cohesion in the writing process, resulting in the plot and character descriptions not being clear enough and lacking in expressiveness and emotional resonance. Writing was a creative process. Everyone's style and characteristics were different. You can improve your writing skills through repeated revision and practice to better express the story and characters.