Rink,
The answer is to say the number of letters used to spell the number the sentry says:
"twelve" has six letters
"six" has three letters
"ten" has three letters but the partner said 5. He should have said 3.
"eight" has five letters so you should say 5 like your partner said only this time it will be correct.
I think it would be better to shoot the sentry as everyone has heard the shot that killed your partner. I am not sure why everyone on both sides is speaking English.
L. Flynn