Well, there are in fact actually a small handful of Pius XII bishops still around.
And the material-formal sedevacantists would have an answer for this.
I'm not sure that the principle that at least one bishop who received jurisdiction from a pope actually holds water, since, as I said, that jurisdiction basically ceases when the pope dies. It does not carry over except through the Church supplying it for the good of souls (i.e. for valid Confessions, etc.) Other than those basic juridical functions, the Church goes dormant juridically (cf. the canon law principle I cited earlier). During any sedevacante, the principle is that the Church supplies that ordinary jurisdiction. That's what I have read about sedevacante periods (the more ordinary ones in the past). I'll try to look it up when I have more time--which may not be until Monday.