The *general* rule of thumb is 8-10 g per liter. That works out to be a generous teaspoon (1 1/4) per 1/2L bottle. Frankly i prefer 4g per bottle a a measure because my scale does grams :-)
All of this is subject to adjustment depending on what the beer is, how you brewed it, the style, etc, etc, yada yada.
Put a slightly generous teaspoon in each. That will (hopefully) carb at 2-2.5