Reading bout United States massacres, most of it was during the civil war; combining them together I'd guess roughly 18k lives were lost in massacres committed by both sides (but mostly the confederates).
Second up you've got the California Genocide - native Americans at ~9k
Conversation
Massacre can be used to describe a specific instance within a genocide.
1
3
It was listed as a massacre in wikipedia; it didn't happen all at once but iirc was a concentrated instance over a relatively short period of time.
Generally speaking, a massacre us a singular or series of events of violence, whereas a genocide is a systematic removal of a social group through violent and non violent means. Forcing a one-language education and compulsory sterilization are both exples of genocide.
1