The common recommended amount of baking powder per cup of all-purpose (plain) flour is about 1 to 1.5 teaspoons. Specifically:

• 2 teaspoons baking powder for every 150 grams (around 1 cup) of plain flour is a common guideline.
• Alternatively, using 5% baking powder by weight relative to the flour is suggested, which is roughly 1 teaspoon baking powder for 100 grams of flour.
• Some sources say 1 to 1.25 teaspoons of baking powder per cup (120-125 grams) of flour is typical.

Overall, using about 1 to 1.5 teaspoons of baking powder per cup of flour is a good rule of thumb for most recipes requiring leavening with baking powder.