You only take an extra prize card if it's specifically written on the card that you do so (Pokemon-ex, Expert Belt, *Salamence LV.X's Double Fall Poke-Power, and most Pokemon LEGEND). There is no such writing on a Pokemon LV.X, so yeah, only 1 base prize card drawn.
*During the turn you play Salamence LV.X from your hand and use the Double Fall Poke-Power, you take 1 more Prize card for each Pokemon Salamence knocks out, on that turn only. It does not apply to taking an extra prize against himself.