"Ich muss meine Mittagessen kochen."
Neither 'meine Mittagessen'. It's just an odd example. How many lunches can a person possibly need? That being said, you might precook your food for the next days, as it used to happen in my family, then they said: Ich muss das Mittagessen der nächsten Tage [of the next/following days] vorkochen.
Lunches can be used in odd cases. Generally it is singular. I imagine because "lunches" sounds terrible, people tend to re-word the sentence like in your example: "I made myself lunch for the next couple of days". Here's an example of lunches:
- "Chef, I made all the bagged lunches for the kids' field trip."
- "How many did you make?"
- "What about the gluten-free lunches?"
- "What do you mean Chef?"
- "I needed 2 gluten-free lunches. 28 regular, and 2 gluten-free."
- "Sorry Chef, I didn't know. I'll get right on that Chef."
- "Stop wasting my time and money Sheffield. You stupid donkey."