They didn't run out of launchers. The launchers just emerged from a predictable place with unfuelled rockets. That made them easy to pot from the air.
Since it took at least an hour for a cruise missile to make it's way and around 15 minutes for an ALBM to make it's way, that means that once Israeli drones started being shot down, they were able to dig out the entrances, exit and launch faster than it took for a munition to be delivered.
Of course if Israel had been able to fly manned aircraft deep into Iran and for prolonged periods of time, that would have been impossible, and they would actually have been unable to fire. But that wasn't the case and so they were able to fire, probably limited by the ability to dig out the entrances and synchronize launches between different sites.
Iranian missiles launchers are pretty cheap, and reportedly quite plentiful - they are relatively simple modifications of domestically produced truck platforms.
The Iranian account for why the strikes slowed down, FWIW, is that it took a significant amount of time to dig out the exits of the missiles bases, not that they ran out of launchers. Given the recycling of footage from launcher destruction and the simplicity of the launchers I personally find that account significantly more plausible.