The first fill on an empty cart (510 model) will usually hold about 11 drops. On average, if I
vape the cart till it starts to "taste funny", I can cram 6-7 drops of
juice into it. If there are about 20 drops per ml (I think thats pretty close) then there are approximately 600 drops in 30ml.
So, let met get my calculator..........about 100 cart refills for 30ml. Thats for me. Your mileage may vary.
If you mean cartomizers, I believe they hold about 1ml each , so you could fill 30 cartomizers. (510 model)