This post is bizarre. There is no minimum number of rockets that can be fired before Israel is justified in trying to prevent further rockets. Who suggested there was? And who said "without casualties"? The question is the relation of civilian casualties to the risk posed by the rockets that Israel is ostensibly trying to stop. In this latest action they are wildly out of proportion.
So how many civilian deaths are in proportion.
You say Israel is justified in preventing it. In order to stop it they need to use at least some amount of force. So the question becomes what is the minimum amount of force needed to defend themselves. There is no proportion. It doesnt work that way. The goal is to stop the rockets.
Now going further, the IDF doesnt just want the rockets stopped, my understanding is that they are now aiming to destroy these tunnels and likely will put off a cease fire until they've achieved that. You can say that's just them still defending themselves or you can say it's not reasonable