"2" being a large number of people to be killed in a crime does not necessarily make it sensible (to me, a Dutchman, very much not American) to call that crime

"2" being a large number of people to be killed in a crime does not necessarily make it sensible (to me, a Dutchman, very much not American) to call that crime a "mass shooting". If the crime was e.g. a bank robbery (sorry for the unimaginative example), and they shot a member of staff and later a civilian to get away, then that's a robbery with two dead, not a mass shooting. What people imagine when you say "mass shooting" is sensational stories from (predominantly) the US where some mad kid takes a gun to a school and shoots around. If that kid shoots 2 people, that's a mass shooting with 2 dead.