Honestly, it really depends on the drawing! But I would say about three. The first/bottom layer is for my flat colours, the second one is usually the line art (if i’m doing a drawing with line art), and the third/top layer is for rendering/drawing over line art and such.
I use additional layers if I want to add more stuff which I might delete. Example; for this drawing I have Kojo’s tattoos on a separate layer for each one rn ‘cause I might change it or scrap it!