Andy is correct. I would like to add just a bit more. How far to reduce a sauce often depends on how it will be used. A full-flavored sauce doesn't always need to be thick. It can be drizzled onto a plate, surrounding the food and adding to the color and presentation. In this case, the sauce should be just thick enough not to run all over the place. The food is then dipped into the sauce and eaten.
Even if the sauce is going to be poured over the food, the thickness of the sauce can vary. If for instance, you are going to put it over a grilled steak, it can be very liquid, like worcestershire sauce. It will still add great flavor. but if you were to put it with a leafy salad, you would want it to have more body so that it can stick to the greens and other ingredients.
You can thicken your sauce with any number of ingredients, such as the afore mentioned cornstarch, flour, butter, and cream. You can also thicken it with pureed fruit, such as plum, peach, appricot, applesauce, raspberries, etc. You can add crushed bannana, or nut butters, or honey. Even sugar will help thicken the sauce.
Of course it also depends on the sauce, and the end flavor you are trying to achieve. My advice is to learn the mother sauces. Make them a few times. Then, when you are familiar with them, you can add other ingredients such as wine or spirits, egg yolks, various starches, fruit puree's, gellatine, gums, etc.
For your next experiment, try this one. Take one can of clam juice, add a pound of cooked salad shrimp, or lump crab meat, and 3 packets of Knox unflavored gelatine. Mix with a tbs. of sugar and 3 cups of boiling water. Stir until the gelatine is completely dissoved. Pour all into a mold and chill until set. Serve on crackers or whole-grain bread pieces along with a bit of cream cheese. Your sauce has become a seafood aspic. Of course, you could add less gelatine and make something more liquid and serve hot as a soup.
The point is, use your imagination. If the sauce is thin, how does that enhance the meal? What can you do with it. Will it make a great aujus? Should you thicken it with a roux, or with cream to use it as a gravy, or make into a soup? Play with the ideas in your mind and you will develop an intuitive sense of how to use various sauces, with differing consistancies to add variety and interest to your meals.
Sseeeeeya; Goodweed of the north