Ricotta is traditionally made from the whey left over after making your main rennet set cheese recipe.
You may find that at times that you get an extremely low yield from your Ricotta make, which makes it hardly seem worth the effort.
There are a few things that will influence the yield you get from your whey including the season the milk is produced in, the level of acidity, the temperature you heat the whey to and the type of cheese you made first.