Suppose the state of Nowhereistan has subjects of different ethnic groups, including, say, a Million Somebodyites. One day, the government decides ceremoniously to randomly choose one Somebodyite and execute them, as a symbolic punishment for the ethnic group's supposed collective conduct. The decision is announced publicly, along with the reason, and everything is minuted and even recorded on video. And - the decision is carried out.
There is definitely an intent to destroy in a very small part of the group, as such: 0.0001% of the group. So - would this action constitute genocide according to the international convention? If you want to make the question a little more concrete, assume an international adjudicative panel of the kind which has typically adjudicated such claims in the past.
Now, if you've answered "No" (and explained that part of your answer), here's a second part of the question for you: If the decision is to execute all 1,000,000, not just 1 - that would surely be considered genocide, right? Ok, so, how would the line be drawn between 1 in a Million and the whole Million? (Obviously, it would not be a specific number, hence the question is how, not where.)