Whenever a creature with flying enters the battlefield under your control, it gains haste until end of turn.
Whenever a Dragon enters the battlefield under your control, it deals X damage to any target, where X is the number of Dragons you control.
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.
If a Dragon entering the battlefield causes Dragon Tempest’s second ability to trigger but leaves the battlefield before that ability resolves, that Dragon still deals damage.
The two abilities aren’t mutually exclusive. If a Dragon with flying (which is most of them) enters the battlefield under your control, both abilities will trigger.