The number will be different in each case. Many years ago, Nintendo said in an interview that they have to sell at least 10 million pieces of hardware for a product not to be a failure (it was around the time of the GB Micro's demise).
But this will be different in each case. Sony sold many, many times more PS2's than Nintendo Gamecubes, yet Nintendo posted profits each year. So the GC wasn't a failure in that sense (I haven't looked at detailed financial results to see if the GBA saved them in those days).
Edit: Nintendo recently said that the 3DS' install base is now sufficiently big enough to release more heavy hitting software, and not worry too much about hardware. So you need 40 millions consoles out there before you can start to rest a bit (hopefully they won't).
Yep.
