How many deaths make a genocide?

Is it a number or a percent?

Does intent play into it?

I'm not sure that genocide is a numbers game, although I would agree that it takes atleast one persons death to create genocide. Actually it would take a few people from the same group to die intentionally for the same reason. I think the main factor in genocide is intent. Its an interesting thought.

gen·o·cide ( P ) Pronunciation Key (jn-sd)
n.
The systematic and planned extermination of an entire national, racial, political, or ethnic group.

I agree with MakeMoneyWemail. I think you don't need a big number of deaths to have a genocide. I would say that a genocide starts with the first death when your intention is exterminating a group of people who have a nationality or race in common.
So yes, it's the intention what counts, if for example you decide to kill black people because they're black, the genocide starts with the first black you kill.

So in other words, Mass Hate Crimes.

Is war genocide too? Or is it self defence?

I believe it would be attempted genocide until the last one was killed.