"The less people know about this, the better."
I don't know exactly, but if someone says "the less people...the better" it's clearly understood that some people already know about it and the speaker wants to avoid a larger number of people to get in touch with this information.
If you say 'the less people..' that means you want people to know as little about it as possible.
If you want something kept a secret do not tell anyone. Many times people will "only tell your secret to one other person!" Only ONE!