Are you one of those people that constantly under or over estimates how much pasta you need for your meal? Leftovers aren't a bad thing but it's better if you plan for that rather than being drawn into serendipity.
So, here it is, definitively: 100g per serving (+20g if you're really hungry & -20g if you're not).
Each 100g will require 1L of water to cook well for 8-10mins.
If you don't have a measuring/weighing implement 100g is a small handful as a really rough measure.