The Numerical Doorman Riddle

James wanted to enter an exclusive club but did not know the password that was required.
He waited by the door and listened to the comments of others.
A club member knocked on the door and the doorman said, "twelve." The member replied, "six " and was let in.
A second member came to the door and the doorman said, "six." The member replied, "three" and was let in.
James thought he had heard enough and walked up to the door.
The doorman said ,"ten" and James replied, "five." But he was not let in.

What should James have said to gain access?