To determine how many you need, take the diameter of your Dutch oven and double it. That’s how many briquets you need to heat it to 350 degrees. So, for a 12-inch Dutch oven, you would need 24 briquets.
Each additional briquet will increase the heat about 20 degrees. For baking, put most of the coals on the lid. For frying, put most of the heat on the bottom. (Source: “101 Things to do with a Dutch Oven.”)