Multiple sources are being cited in the various reports. We may never know the exact death toll. It’s likely that many of the bodies are not intact. And the Muslim custom is to bury their dead immediately, so many of the bodies may not be taken to a morgue to be counted.
Well I’m not familiar with Yazidi funeral traditions, but the victims weren’t Muslims, but as already said Yazidis, an ancient pre-Islamic religion.
Of course we’ll never really know just like we never really know about all these bodies dumped every morning in Baghdad that no one ever sees.