There are many ways that you can change the number of servings. First, you can increase the number of family members in your account. Click "Account Settings" in the triple-bar icon in the top navigation bar. Then click on "Household" to add additional family members, which will increase servings.
Second, you can adjust the servings for a particular meal, too. There are several ways to do this:
Click the ellipsis icon on a meal tile and select "Batch Cook"
Click the ellipsis icon on a meal tile and select "Change Servings"
Click on a meal tile to view the recipe and then click "Serves X" in the top corner
All of these options will allow you to access the following menu where you can adjust the number of servings.
These options will only adjust the servings for that particular meal.