According to the FIDE rating regulations -
5.1 Whether these occur because of forfeiture or any other reason, they are not counted. Any game where both players have made at least
one move will be rated.
It looks like "resigns" is not a move. Games will only be rated if each side has moved at least one piece.
I believe this regulation was introduced after an incident in the last round of a big weekend tournament in England in the mid 1970's. Tony Miles was leading the competition and only needed a draw in the last round to guarantee sole first place. He met his opponent, Stewart Reuben, in the bar the night before and agreed a draw. The next day they played no moves and simply wrote on their scoresheets "draw agreed". Tony Miles won first place and Stewart Reuben shared second with several other players. They were given their prize money and everybody was happy until somebody noticed they had played no moves. Then the organizer was unhappy. The result between Miles and Reuben was cancelled and they were required to repay their prize money. Now this practice is explicitly banned.
If you look in the FIDE Laws of Chess you see that the full title of article 3 is -
Article 3: The moves of the pieces
From this you can also see that in chess moves are moves of pieces. "Resigns" is not a move of a piece and is therefore not a move.
So, the correct answer is 20.