Preheat the oven to 350°F. Keep 6 shrimps whole and chop the remaining 20. Grease an oven-proof dish and spread a thin layer of béchamel sauce; cover with 3 Lasagna sheets. Top the Lasagna sheets with a layer of Marinara Sauce, evenly distributed, then top with chopped shrimps and béchamel sauce. Repeat the steps to form three other layers. Top the last layer with whole shrimps, béchamel sauce, and grated Parmigiano Reggiano cheese. Bake until golden brown for around 20 minutes, slice and serve.