According to Vgchartz, Okami has sold 150k in Japan. So let's add that:
Japan: 150k
And according to this source, Okami sold 270k by March 07, so with legs it might have sold 300k in America
America: 300k
And while the game was completely ignored in Europe, a large userbase must have gave it some sales. Let's go low and say 50k.
So, what we end up with is...
Japan: 150k
America: 300k
Europe: 50k
Total: 500k