Bring a large pot of salted water to a boil and cook the pasta to al dente, according to package instructions. Reserve 1 cup of the pasta water, drain pasta, and set aside.
Melt the butter in a large saucepan over medium heat. Once hot and sizzling, add the flour, then whisk constantly until it turns a golden color and smells nutty. Add the garlic and cook for 30 seconds. Slowly pour in the milk a few splashes at a time, whisking constantly to smooth any clumps. Increase the heat to medium high, then let cook, stirring constantly, until the sauce bubbles and thickens, about 8 to 10 minutes. Remove the pan from the heat, then stir in the pumpkin, cream cheese, half of the chopped sage leaves, salt, pepper, cinnamon, nutmeg, and cayenne. Continue whisking until the cream cheese melts and you have a smooth, rich sauce. Taste and add additional salt and pepper as desired.
Add the cooked pasta to the pan and gently toss to combine. Thin the pasta with a bit of the reserved pasta water as needed if it seems too thick or sticky. Serve immediately, topped with Parmesan and the remaining sage.
This recipes serves 2 generously or 3 with smaller portions. To make this a heartier dish (and extend the servings to 4), try adding sautéed greens such as spinach or kale, shredded or chopped cooked chicken, or sautéed chicken or turkey sausage.
I have not tried doubling the recipe but don't see why it wouldn't work—just make sure you use a large enough pan.
Reheat leftovers gently in the microwave with a splash of milk to keep them from drying out.