Corrected entry: There is a scene in the training area where Gobber is counting the shots from the gronkle, mentioning that there is a 6 shot limit. At the end where Toothless is fighting the large dragon there are at least 10 shots fired.
spiderheiney
16th Jul 2014
Correction: He mentions that each dragon has a limited number of shots and that each is different. The Gronkle has 6 shots while the number of shots for a Night Fury is still unknown.