As per Israeli intelligence, Iran still has 1,500 to 2,000 active and available missiles that can reach Israel. and 6,000 - 8,000 more that are temporarily stuck inside the locations Israel attacked. So Iran has more than enough missiles. And that's Israeli intelligence, not Iranian.
Its the launchers that are an issue. Israel is said to have taken out 40% of Iranian launchers in just 2 days. If Iran loses launchers then all of those missile will be useless. so Iran is now conserving the launchers, taking them out in small numbers, hitting and puling them back. Having a few launchers makes it less risky that a larger number will be taken out.
Once Iran regains the airspace, then they can start launching in larger frequencies as you suggested..