Nah, he mouthed ten. It'd make no sense for him to mouth six when the officer had already said six.
This shows a lack of understanding of the scene and general human expressions. He repeats what the officer says(mouthing the word six), as if in surprise for comedic effect.
You can't say it's ten/six simply because it looks like it's ten/six, that is invalid. It's pretty easy to say ten, or six, and have the movement of your mouth stay the exact same.
In the context of the dialogue and characters, ten makes no sense what so ever.