please Before adding the question, just check is it asked or not?
duplicate of https://gateoverflow.in/16808/many-state-drawn-over-alphabet-which-accepts-empty-language
here we will have 2 cases :
CASE 1 : when we donot have any final state means each symbol can goto any state means each symbol has 2 choices for any state.
CASE 2 : when we have 1 final state but its not reachable. so final state will have 2 choices of states to move on to for each symbol and initial will have only one choice.. so answer is 20 as initial state is fixed.