Because recipes list ingredients with various types of measurements (weight, volume, and so on) there is no way for the recipe editor to calculate how many servings of a particular size will result when the various items are combined.
If you're not sure how many servings a recipe will produce, enter a best-guess number when you first create the recipe.
After the recipe is prepared, you can manually measure the number of servings that have been produced. Then, the recipe can be edited and the correct number of servings can be entered.
We would also recommend then putting in the actual serving amount in the title of the recipe to then remember how much each serving weighs.