Well, given that nobody else has jumped in, I'll stick my toes in the water.
The idea of document control (as well as everything else) is to ensure you are able to deliver quality products or services. One aspect of doing so is to ensure that information is properly managed. Continuing to drill down, one aspect of this is to ensure that you control information in the form of documents.
So not every document your company has need be subject to document control. An internal memo, for example, suggesting that you get a raise - while very important (especially to you) - probably doesn't need to fall under document controls. You have to look at your processes and determine what documents do impact quality. These will fall under document control. Thus, I expect that you haven't received much feedback on your query because nobody in the forum can say whether or not an individual email does or does not have an impact on quality.
As far as numbering goes, there are no specific requirements. Documents have to be uniquely identified. How you implement that is up to you. A common theme you'll see as you browse the cove is to do what makes sense for your business. Same goes filing.
One of the keys to document control is to ensure that everyone is using the proper revision. So if you send a controlled document to an external party, you need to keep track of it. If that document changes, you need the processes in place to ensure that the external party gets the updated document.
This is pretty much only scratching the surface. You need to get the standard and begin formulating plans how you will implement it. As you run into specific difficulties, you should come back to the cove with questions. Folks are generally quite helpful when you ask specific questions. It's more difficult to answer generalities.
Good luck!
