One, I would imagine, considering that the Wii-U is considerably more powerful than either the 360 or the PS3.
Until the Sony/Microsoft release their new console in the future rendering the Wii-U redundant and third party developers abandon it... again, like they more or less did with the Wii.
Nor does a powerful console mean it's only going to take one disk, the main reason Sony has a step up on Microsoft is because of the Blu-ray storage advantage over the DVD. There'd be little between the 360/PS3 if it weren't for that. I somewhat doubt the Wii-U will be using Blu-Ray or any significant storage disk so regardless of how powerful it is, it's going to be hindered. As much as Nintendo hype this thing up, it's still 6/7 years behind the times.