What's the password?

Question

War has broken out in the kingdom. The king's castle is guarded pretty heavily, by guards with instructions to kill anyone who can't produce the correct password.

Unbeknownst to the guards, the beggar near the main gate is in fact an enemy spy. He listens intently as the first knight approaches. "Six!", the guard calls out. "Three," answers the knight. He is allowed through. "Aha," the spy thinks to himself.

But he needs to be sure. He sees another knight approaching and listens in on his exchange with the guard. "Twelve!" "Six!"

The spy leaves, and returns dressed as a knight. "Eighteen!" The spy answers "nine" and is killed on the spot by the guards.

Why?

Answer 1

The kingdom is guarded from foreigners, therefore the password is based on native language (which is of course English:):) ).

You just should tell number of letters in the word, that is told to you. And "Eighteen" has 8 letters, not 9.

Answer 2

Hint:

The division is simply a ruse.

This appears to an answer:

The guards say the number of letters in the word.
S I X
1 2 3
T W E L V E
1 2 3 4 5 6
So in the spy's case:
N I N E T E E N
1 2 3 4 5 6 7 8

RIP enemy spy.