Does the effect of Buster Blader power up for both dragons on opponents field and graveyard or only one is targeted because it says gains 500 OR graveyard?

Buster Blader's effect only targets one dragon at a time.

The wording "gains 500 ATK for each Dragon-Type monster your opponent controls or in their Graveyard" indicates that you choose one option:

* Option 1: Gain 500 ATK for each Dragon-Type monster your opponent controls.

* Option 2: Gain 500 ATK for each Dragon-Type monster in your opponent's Graveyard.

You cannot choose both options simultaneously.

You need to choose one option and apply the effect to all the Dragons that meet the chosen criteria.