If I have multiple Rapidash in play can I use each one of their "Heat Boost" Abilities once per turn, or do I only get to use a maximum of one "Heat Boost" per turn regardless of how many I have in play?
Each Rapidash can use its own Heat Boost during the turn, but only one time each. And when you use Heat Boost multiple times you have to discard a separate 
Energy card for each one and your Pokemon's attacks do +30 more damage per Ability used.
Heat Boost