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.
Thank you so much Arjun Sir!