The premise of Star Trek: Voyager is that the Voyager is far from Federation space, and operating with limited supplies. The ship starts the series with 38 photon torpedoes and "no way to replace them." Okay, so how many photon torpedoes are actually fired by Voyager in the show? Watch and learn:
Because the Internet is awesome, lots of people have already done the math on this, though they tend to give up in frustration after a while (unlike the intrepid video editor above). The short version is that the writers did keep track of the photon torpedo count until the fifth season, then decided they needed a way to make more. And eventually they ditched the count entirely, because who really cares? Anyway, you can read a brilliant writeup here that attempts to tie together all the threads of this very nerdy problem. Another useful resource is Rick Kuzma's Star Trek: Voyager Statistics.
"We need more firepower," indeed!