There is a number riddle at the party, and the answer is an 8-digit riddle. Now I have guessed it: □54□7□39, and the host prompted: "This 8-digit riddle with no duplicate numbers.

The smallest number in this octet that is not complex is 2, so according to the meaning of the question, you can only choose three boxes from 2, 6 and 8. According to the principle of multiplication, you have to guess 3×2× 1=6 times at most.

So the answer is: 6.