1,000,000 bones, manufactured by 100,000 people, over three years. That's 10 bones per person. Each person making one bone every 109.575 days.
1,000,000 bones, spread by 1,000 volunteers, clad all in white, across the National mall in four hours. That's 1000 bones per volunteer, 240 minutes in four hours, so each person laying down a bone every 14.4 seconds for four hours. Picking up bones, moving to an empty space, putting down a bone every 14.4 seconds.

There are 206 bones in the human body.
Wikipedia has a List of genocides by death toll.
The United Nations Convention on the Prevention and Punishment of the Crime of Genocide (CPPCG) defines genocide in part as "acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group". Determining what historical events constitute a genocide and which are merely criminal or inhuman behavior is not a clear-cut matter. In nearly every case where accusations of genocide have circulated, partisans of various sides have disputed the interpretation and details of the event, often to the point of promoting different versions of the facts. An accusation of genocide will almost always be controversial. Determining the number of persons killed in each genocide can be just as difficult, with political, religious and ethnic biases or prejudices often leading to downplayed or exaggerated figures. Some of accounts below may include ancillary causes of death such as malnutrition and disease, which may or may not have been intentionally inflicted.
Let's consider a relatively recent, fairly well documented, moderate genocide from the list:
Rwandan genocide, Rwanda, 1994, low estimate 500,000 killed, high estimate 1,000,000 killed
That's a low estimate of 103 million bones, high estimate of 206 million bones, produced in one year by the slaughter of Tutsis by Hutus. Most of the bones in the human body are smaller than the clay or paper-mache model bones manufactured for the One Million Bones project, so the occupied space would not be 103 to 206 times the size of the National mall. However, a 1000 volunteers would still probably spend 17 to 35 days spreading out those bones, one bone every 14.4 seconds.
Or take the biggest listed genocide, the
Holocaust, with low estimate 4,194,200 killed, high estimate 17,000,000 killed. A low estimate of 800 million bones to a high estimate of 3.5 billion bones. The 1000 volunteers would spend somewhere between 135 days and 583 days spreading those bones.