Okay, more info, got it. I can relate to this, however, my situation has differences versus yours, so take my advice with a grain of salt. I can give you three different answers, it really depends on how you want me to respond. I have an optimistic, realistic, and pessimistic response depending on what you feel you need to hear.
Optimistic (hopeful)- Your parents will resolve the argument and stay together for one reason or another. They'll make sure you know this too, and they'll calm you down. You'll be okay!
Realistic (probable)- You said your parents have been married for a long time. They likely would not split up, since divorce is a lot of paperwork and dividing of properties and belongings, and of course, custody which they would most likely split. Even if they fight, most couples will, for some reason or another, find a way to push on and make the relationship work, or they will just have the argument and agree to disagree on that matter. For an example of a reason, making sure you have a stable home. My parents fought for a long time, and while they didn't get along well, it took a lot for mine to ever split up. Besides, my parents never married and never had divorce paperwork to go through when they split, it was peaceful.
Pessimistic (in preparation for the worst)- The argument worsens and they do split up. There will likely be one of them moving out and a bunch of stuff with dividing everything. I don't know you're relationship with your parents, but they'll probably have split custody, so you'll still see both of your parents and will go back and forth every so often. One thing I recommend keeping in mind if your parents split, it is not your fault and the split will be because of a division in their relationship, your relationships with your parents remain the same. But if they split up, it will be chaos for a little while, just try to get through it the best you can. You may have to be emotionally self-sufficient for a little while. The chaos will dissolve after about a few months to a year at very most. You will be okay, things will change, but in the end, your parents will be happier if they decide that divorce is their best option, and they likely won't choose to unless they feel there is absolutely no way to fix their relationship.
