Commercial "carbonation drops" (pre-measured dextrose tablets sold for exactly this purpose) are usually dosed at one drop per standard 12 oz (355 mL) bottle, but whether that's the right amount for your batch depends on your actual target CO2 level and how much residual CO2 your beer already carries — the same variables every priming calculation on this site depends on.
Working out the equivalent dose from a real batch
A 19 L batch at 68°F targeting 2.5 volumes CO2 needs roughly 125 g of corn sugar total — check this batch total here. Split evenly across roughly 53 standard 355 mL bottles (19,000 mL / 355 mL), that works out to roughly 2.4 g per bottle.
Why that lines up with "one drop", roughly
Commercial carbonation drops commonly weigh somewhere in the range of 2-3 g of dextrose each, sold as a "one drop, one standard bottle" convenience product. Compare that to the roughly 2.4 g per-bottle equivalent computed above from a real batch-priming calculation, and a single commercial drop is dosing right in the same range a full batch-priming calculation would put in that same bottle for a 2.5-volume target — which is exactly what you'd expect, since drops are designed to approximate a standard ale-strength carbonation target for an average bottle size. A style targeting notably more or less than 2.5 volumes will still diverge meaningfully from a fixed one-size drop, which is the case batch priming actually handles better.
Why batch priming is still the more reliable method
This arithmetic is a useful sanity check on what a drop is actually providing, not a recommendation to dose bottle-by-bottle with loose sugar instead of drops or batch priming. Dissolving your full calculated sugar dose in a small amount of water and gently mixing it into the whole batch before bottling — standard batch priming — distributes carbonation far more evenly across every bottle than either individually-dropped tablets or hand-measuring loose sugar into each bottle one at a time, where small measuring errors compound across a full case. If you do use commercial drops, check the manufacturer's stated CO2 target for their recommended dose against your own style's target — see CO2 volumes by style — rather than assuming "one drop" is calibrated for whatever you're brewing.
If you're bottling a genuinely small batch
The per-bottle math above scales the same way for a smaller total batch — see how much priming sugar for 1 gallon for the full small-batch version of this calculation, where accurate weighing at a small total dose becomes the bigger practical challenge than the arithmetic itself.