At least 500k, but I think 750k is a possibility.
PS2 sold 270k and Capcom is happy with that level of sales on Wii. Though I think Okami appeals more to a Nintendo crowd in the first place. I think/hope a lot of Zelda fans will buy it.
But 270k is a guarantee. Both niche games Z&W and NMH sold more, and Okami is an epic adventure.