Assuming you mean on a full-size keyboard, I mostly use 6 – thumbs, index, and middle fingers on each hand, with an occasional ring finger (right more likely than left) thrown in. I went to High School in the early ‘80s. Typing was an optional course that was mostly targeted to those girls who were not likely to go to college. Computers were still very new, and it was an elective class in which we learned MS-BASIC on an Apple ][e. I tried a couple self-taught typing courses over the years, on an old IBM typewriter and on the computer, but never did well. My visual memory and fine motor skills are inadequate. I have to look at my hands.
On my touch screen tablet I might use index and middle fingers. On the smart phone, I use right index and sometimes middle finger. When the iPhone first came out, I was pretty good at using the index for the left side of the keyboard and the middle finger for the right side; but I guess I type less on it now than I did back then, because I seem rusty when I try. I often use voice dictation on these devices nowadays – very convenient, once you get used to verbalizing your punctuation.