How much water do I use for lentil soup in slow cooker?

Bistro Lentil Barley Soup
Serves/Makes: 10

Ingredients:
1 bag (16 ounce size) brown lentils – sorted and rinsed
2 medium onions – chopped
2 medium carrots – diced
1 can (14 1/2 ounce size) diced tomatoes with roasted garlic – undrained
12 ounces smoked turkey kielbasa cut lengthwise into fourths, then sliced
1/3 cup uncooked barley
2 cans (14 1/2 ounce size) ready-to-serve beef broth
3 1/2 cups water
Dijon mustard – if desired
Prepared horseradish – if desired

Directions:

Mix lentils, onions, carrots, tomatoes, kielbasa and barley in 4-quart slow cooker. Stir in broth and water. Cover and cook on low heat setting 9 to 10 hours or until lentils and vegetables are tender. Top each serving with dollop of mustard and horseradish.

This recipe from CDKitchen for Bistro Lentil Barley Soup serves/makes 10

You should use about 3 or 4 cups of water (or even better, chicken stock) per cup of lentils. A pound of lentils should yield about ten servings.