Flameblast Dragon has a triggered ability that triggers whenever it attacks. When this ability goes on the stack, you choose a single target creature or player. When this ability resolves, you can pay one red mana and any amount of generic mana. If you do, the targeted creature or player will be dealt damage equal to the amount of generic mana you paid.

This ability will only trigger once each time Flameblast Dragon attacks. You cannot pay one red mana plus X as many times as you like in order to target and deal damage to more than one target creature or player just because the Dragon is attacking.