The general rule of thumb with the baking powder to flour ratio is 1 to 2 teaspoons for 1 cup of flour. Baking powder should not be confused with baking soda as the latter is four times stronger than baking powder.
Baking powder is a combination of baking soda, one or more acid salts, such as cream of tartar and sodium aluminum sulfate, and cornstarch to keep the mixture dry thus preventing any chemical reaction until liquid is added. Adding too much baking powder to a batter will leave the bread or cake bitter tasting. The extra baking powder may also cause the dough to rise too fast then collapse. Using too little baking powder will make the cake tough and have a compact crumb.