regardless of whether Judas was a "true" disciple - he did according to the narrative hang himself and thus removed himself from the ranks of the disciples. So I would say you are safe to continue with 11 balls on the simnel cake. Or - since you don't "do religion" you can decorate it as suits your fancy. 11 white balls and one black one? 13 balls to include both Judas and whassisname they elected to take his place? 14 to include the above 13 plus John the baptist?

24 to be the disciples and the 12 major prophets of the Torah?