A club member walks into a club and before he enters the security guard tells him six and he replys 3 and he gets let into the club, anorther club member comes in and the security guard tells him eight and the member replys 5 and he gets let into the club, A bystander who watched the other 2 members get let in comes to enter the club and the security guard tells him 10 and he replys 7 but doesn't get let in.
What should he have said? and why?
Clue : letters
first person to crack this gets 1M 07