In the manga, Sukuna doesn't possess all 20 fingers. There are specific story arcs and events where his finger count changes based on the plot developments.
In the manga, Sukuna does acquire all his fingers over the course of the story. This is a significant plot development that has major implications for the storyline and character dynamics.
Well, start by understanding the basic structure of fingers. Observe real hands and notice the proportions and joints. Practice sketching simple finger shapes first.
Honestly, I don't have the exact number of fingers Itadori has eaten off the top of my head. It depends on the detailed plot developments in the manga. You'd have to go through and carefully follow the story to figure it out accurately.