Enough for Microsoft to post Xbox One shipments again to brag about them.

The X1 would be a sucess if it sold in the same ballpark as the 360.

As it's looking, it will represent about a 50% market loss at the end of the gen. There is no way to spin that into a sucess.
There may be profitability though. Wich is a different thing.

I think somewhere between 50m and 60m is a great number for a console! :) It should easely reach this with the scorpio and a new halo game and some good PR from backwards compatibility and improvements phil and his team have reached the last few years.

I will be joining the Xbox family with the Scorpio and when I bought my PS4 at launch I had no expectation to ever want an Xbox and I am sure there will be others like me who want to have both consoles after the excellent decisions Microsoft has made over the last few years.

Its also really good value digital games can be shared, xbox live gold membership can be shared and digital purchase also include a PC version of the game!

50M+

I'd say around 60m, I can't view a decline in install base of more than 30% a success.

But 50 Million is still a respectable and fine number, just not quite a success for me.

If it doesn't hit 50 million in the next 4 years, I would be very surprised.
50 million units isn't a failure by any stretch of the imagination... And it could even better that.

50 million is a very very good and super respectable number. No one can be upset about selling 50 million units.

35 million.

I don't think it's a failure, but I'd definitely bet it is an internal disappointment for MS. Still, it should go on to do above 50 million, which is quite good. I expect Scorpio to give it a decent boost this year. 

50M is Ok, but we need to know from the company's perspectives if It's Ok
Companies some time look at the successor to achieve at least the same numbers the old one managed, if this is the case then 90M is what they are hoping for