Each battery has 3 launchers with 20 missiles each.  7 batteries X 3 launchers X 20 missiles = 420 Iron Dome missiles vs 40,000.  That's about 1% coverage.  Even if 'only' 4,000 missiles are launched (as in the last 'war') that's 10% coverage.

 

Iron Dome is too expensive to be used against Quassam rockets (very poor ROI given the Quassam's gross ineffectiveness as an actual weapon - they are more like reminders that Hamas is still alive...)

 

Iron Dome is probably thought to be useful against large ballistic missiles coming from Iran (after an israeli strike) or Syria.  OR the rogue BM-21.

 

IRan and its buddies are smart enough to know that if Iron Dome missiles are used against dirt-cheap BM-21's, those will be launched piecemeal until the anti-missile missiles are depleted.  As such Iron Dome is like a vert small fire in a large winter storm.  Give more hope than heat.