Rulings
The amount of damage dealt by the Dragon that entered the battlefield is based on the number of Dragons you control when the ability resolves.
(wotc • 2017-11-17)
If a Dragon entering the battlefield causes Scourge of Valkas's ability to trigger but leaves the battlefield before that ability resolves, that Dragon still deals damage.
(wotc • 2017-11-17)
Rulings
The amount of damage dealt by the Dragon that entered the battlefield is based on the number of Dragons you control when the ability resolves.
(wotc • 2017-11-17)
If a Dragon entering the battlefield causes Scourge of Valkas's ability to trigger but leaves the battlefield before that ability resolves, that Dragon still deals damage.
(wotc • 2017-11-17)