Hello Kristen. For a standard, hearty bowl of soup, a good rule of thumb is to measure out 1/4 cup (about 50 grams) of dry lentils per single serving. If you prefer a very thick, stew-like soup, you can increase that slightly to 1/3 cup (around 65 to 70 grams) per portion. Keep in mind that lentils will expand to roughly two to two and a half times their dry volume once they have finished cooking.
To ensure your soup does not turn into complete mush, especially when meal prepping for the week, there are two key factors to keep in mind:
First, choose the right variety of lentil. Brown lentils, standard green lentils, French green (Puy) lentils, and black Beluga lentils retain their structural integrity quite well throughout the cooking and reheating process. Avoid red or yellow split lentils for whole-lentil soups, as they naturally break down and disintegrate into a smooth puree.
Second, control your cooking temperature and time. Cook your soup at a gentle simmer rather than a rapid, rolling boil, since violent movement in the pot will physically break the skins apart. Start testing the lentils for doneness around the 20 to 25 minute mark. As soon as they are tender yet still have a slight bite, take the pot off the heat. Cooling your batch quickly before refrigerating will also prevent residual heat from overcooking them.