There are several factors to consider when determining how much frosting you will need. One factor in determining the quantity of your frosting recipe will depend on whether you need frosting for cake decorating, frosting for cupcakes, or for frosting cookies or bars.
If you are trying to determine how much frosting you need and are questioning whether you will need additional or not, you should probably go ahead and make more. The last thing you want to happen is to get almost done decorating your cake, cupcakes, or cookies and then run out of frosting before you are finished. It would be better to end up with leftover frosting than to run out before you are done decorating. If you have leftover frosting, many times the frosting can be stored for a few days and some frosting types can be store in the freezer to use at a later date.