Depends.

For a company's first console 20M could be seen as a sucess.

For either of the big three, I would count anything under 50M lifetime sales as a failure as last gen all 3 consoles were well above 50M.

At the same time, this is specifically about failure.  You can not fail, but at the same time still not succeed.  

Failure, as I think of it, is failing to profit from your product.  Even if you sell well, not turning a profit in the end hurts.
But of course, the installed base of your system matters to other companies because a higher installed base means better chances of seeing results from investment.

30,000,000.
12,000,000 in N.America (10,500,000 in USA)
9,000,000 in Europe.
6,000,000 in Japan.
3,000,000 in rest of world.

Es:
Nintendo 64 have sold good W.W, over 30,000,000.
But is failure in Europe and Japan.
GC have sold bad W.W... but in NA have sold very good!!!

PSVita?

If you're a company I'd imagine profit is what determines whether or not it was successful. For your average internet guy the number of total sales is what really matters (unless you invest stock into Sony, MS, or Nintendo). But to answer your question I'd say in today's market 40 million is a success, maybe even 30 million.

It depends on what the goal of the company is for that specific product at that time -   Amazon lost tons of money for years in order to expand market share - it was a succesful strategy -  they could have turned a profit sooner but it would have slowed thier growth

when MSFT launched the 1 st xbox they cared about market share much more than profit 

right now Sony has said market share is the # 1 goal for the PS4 ( at this time)-   

If the Wii u breaks even or makes a small profit yet gives up huge market share it will be consider a failure by Nintendo especially coming off the Wii.   

a failure in the plublic's eyes? 50 million. I wouldnt say 20 million because of the dreamcast.

After Deliberating with my counsel, I've devised a fool proof measure of success.

successequation = (profitability(per dollar)*marketshare(LTD per unit)) - (average unit price)*(marketshare LTD per unit))

successequation > 0 = success

successequation <= 0 failure

In order to consider the competition: successequation + e^^{[MarketShareGap(Next highest competitor price per unit)]}

unfortuantely we don't have figures on profitability...

The amount of Vitas sold+1, the console is intended to be a failure.

@Bold:  Except the goal they were going for was expanding their market share.  It's different if they released a product meant to reel in profits, but instead shows a net loss.  


@Italics:  That would most likely be due to the fact that they wanted to participate in the big game, so getting known was more important than making a profit for the time being.  However, that was set up for their successor.

@Underline:  Losing portions of the market share (whether large or small) isn't ideal, but when it comes to this topic, making a profit is more important than how much of the market share they have any single generation.  What your console had the previous generation vs what your generation has now aren't that largely affected by each other.  If they were. the sales of each console generation should reflect it more accurately.  Fact is, it doesn't.  However, that market share does mean something, but losing it doesn't mean failure for that console.  If they actually maintained their portion of the market (percentage wise), that would at least be successful, but isn't required.