Missouri has a law that allows citizens to defend themselves in public areas. There is not a duty to retreat. Sometimes referred to as a Stand Your Ground law, in Missouri, if you have a legal right to be in a location, then you also have a right to protect yourself if you’re in imminent threat of deadly harm.
That law also extends to the protection of others under the same philosophy. If they’re under imminent threat of deadly harm, then you can protect someone else. However, you cannot be in the course of committing a different crime (for example, you can’t be trespassing), in order to invoke the Stand Your Ground law.
Missouri allows you to defend yourself with the use of deadly force if you’re under imminent threat of deadly force, without a duty to retreat in public. However, there’s not a rule that says that you need to be 21 feet away from the aggressor or that the aggressor needs to be within 21 feet of you. That simply does not exist.
It’s important to remember that not all jurisdictions have a Stand Your Ground law. Missouri does and there are many states that do. If you’re traveling, be sure that you understand the laws of the state in which you will be.