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Don't aim for that many missions. Hell, don't aim for a specific number. Tell your story, cram as much gameplay and story into every one - You'll end up with far fewer missions, but generally much more "fruit filled" ones. It also helps to use a notepad and write down (yes, pen and paper) your story. This also helps you maintain a sense of internal continuity. It's also good to actually preplan every mission by writing a script, a kind of "general rundown" of what you have to do. Once you get to a certain script, you know where your going - Then you can explore it in more detail. Don't fragment your mission building, but try to have a balance of arc/standalone missions. The arc missions would be part of the major story, the standalones would be fillers with small arc elements.