In theory, you need 8 questions. In practice, you could get lucky.

After 6 questions, you should be down to 4 choices. You then have a 1/4 chance of guessing which. The seventh question would narrow it to two, with no questions left.

Note that using 7 questions and guessing (50-50 hance) might get the correct number, but you would not know if it were the correct one.

If you had 256 doors, with a prize behind one. Asking 7 questions and guessing which of two doors would be your best strategy.