It was derived from the term 10-12, a CB radio code which means that visitors are present and one should be discreet. 12 is the police. Shouting TWELVE will warn the presence of an officer approaching the scene of some illegal activity.
CB radio terminology and 10-codes are code words used to represent common phrases in voice communication, particularly by law enforcement and in Citizens' Band (CB) radio transmissions. The use of 10-codes originated in the early 1920's, when public safety radio was in its infancy and officers rarely had to communicate with officers outside their own department.
Here are the most commonly used 10 codes are:
10-1: Receiving Poorly
10-4: Ok, Message Received
10-7: Out of Service, Leaving Air (you're going off the air)
10-8: In Service, subject to call (you're back on the air)
10-9: Repeat Message
10-10: Transmission Completed, Standing By (you'll be listening)
10-20: "What's your location?" or "My location is..." Commonly asked as "What's your 20?"
If you’re not sure whether the situation is a true emergency, officials recommend calling 911 and let the call-taker determine whether you need emergency help. Check out some guidelines on when to call 911 at 911.gov.