I don't know. That would depend on whatever number is necessary for it to register in a statistically significant way.

Tongue in cheek aside, it's incredibly stupid to make sweeping, broad, categorical judgments of certain things because of isolated, exceedingly rare incidents. It's even dumber to make broad, sweeping laws around emotional reactions such as yours to such events.

Every school shooting is a grotesque tragedy, but means of fighting them exist that don't require gun ownership to be banned for the millions of people who not only peacefully use guns but also in some cases need them for an assortment of reasons. "What about the children?" has rarely been a good or honest argument for anything, particularly for badly considered laws pushed forth by bouts of moral indignation and self-righteousness.