The answer is 13.
To explain that, let me answer the question for 27 signals, which is clearer because 27 = 3³.
You form one level of 9 AND gates, each taking 3 of the input signals.
The second level will have 3 AND gates, each taking three of the 9 outputs of the first level.
The last level will have 1 AND gate combining the three outputs of the second level.
For the original problem of 26 input signals,
you use the above solution and tie one input of the first level to constant 1.
(This is assuming that two-input AND gates are unavailable.)
