At the point in which a deliberate strategy or policy is developed to eliminate a population through any means that involves killing, and that strategy or policy is put into practice, that is the point at which a genocide becomes "in progress". However, at that point it cannot be said that a genocide has been committed. Only that one is in the process of being committed.

I don't know how many people need to be killed in order for it to be described as a genocide that has been committed. But I don't think anyone would say that Hitler had committed genocide had he only succeeded in killing a few thousand Jews before being stopped. Nevertheless, while he was killing them, it would have been accurate to say that he was in the process of committing genocide. Remember the definition of genocide...

Genocide is the deliberate and systematic destruction, in whole or in part, of an ethnic, racial, religious, or national group.

Hamas is not trying to eliminate the category of people known as Jews. They are trying to remove the military occupation from their country. The government of Israel, on the other hand, has been in the process of eliminating Palestinians as a people since before Israel became a state. They have been doing it both in terms of ethnically cleansing them, as well as through a deliberate denial of their existence and history. And now, since they are deliberately denying the people in Gaza the most basic fundamentals of the ability to sustain life, they are also in the process of eliminating the Palestinians as a people through deliberate killing. That is a genocidal act even though at this point relatively few people have so far been killed.