SPLC might have a figure for hate based crimes. But you won't know how many of the hate crimes were by members of extremist groups.

You won't know. There are a LOT of extreme rightwing groups in the US.

Do we know how many they killed in previous years? How similar do the groups need to be? Do even more extreme groups like Atomwaffen count?

@TheAllMemeingEye Atomwaffen, Identity Evropa etc definitely count. Otherwise similarity is a judgement call. I don't know how many they killed in prior years.