The quantity of the stevia will depend on the type of product you use. Most extracts have a ratio to sugar as follows:
1 cup sugar = 1 tsp. liquid or powdered stevia
1 Tbsp. sugar = 1/4 tsp. powdered (6-9 drops liquid stevia)
1 tsp. sugar = a pinch of powdered (2-4 drops liquid stevia)
Keep in mind that there are other types of stevia extracts with different ratios, for example 1:1.
Eventually, experiment and see what quantities suit your taste best!
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