How many 1 3 cups make 1 cup?

Techically speaking, 3. ;-)

If you are looking for substituting the milk with yogurt, you need to remember the consistancy of yogurt is quite a lot thicker than milk. I would suggest you use 3/4 cups of yogurt and add water to 1 cup. See if that is still the same consistency of milk. Ratio 3:4. I substituted yogurt equally to milk one time, and had to add water to the mixture. It does create a different texture than what milk would be. Very good flavour though.