If I have X Harmonic prodigy’s, and then cast a instant or sorcery, how many times does prowess trigger on each one?

I was gold-fishing my {{Rionya, Fire Dancer}} deck and made 8 copies of {{Harmonic Prodigy}}. I then casted {{relentless assault}}, and couldn’t for the life of me figure out how many times prowess was supposed to trigger. It would be nice to know the formula, for future reference, along with an explanation for why it happens that many times.