How many teaspoon or tablespoon of brown sugar am i allowed to put into my greentea to avoid removing its benefit?

Less is best: There's no exact answer. It's clear green tea has many health benefits. It's clear that sugar (brown or white) is rather toxic. At which point the harm from the sugar outweighs the benefits of the tea isn't certain and will vary from person to person but the less sugar the better- I advise 1 tsp or less per cup. Even better, use natural safe low-calorie sweeteners like stevia and/or erythritol.