Ok. So Sony announced that they sold 2.1mil as of 01/DEC. 

And then they announced they have sold 4.2mil as of 28/DEC. Which means 2.1mil were roughly sold for December, and we also know that for most of December, ps4s were sold out.

Now come January 20th, PS4s are still sold out as of today. Which means for Jan 2014, we will assume ps4 sales could be something like 2mil again. (Unless Sony for some weird reason is making PS4s slower than they were making them in Dec.)

For February 2014, PS4 debuts in Japan, where pre-orders are said to reach 1million. So for February,  we will assume ps4 sales are around 2 mil again. 

For March 2014, lets say it slows down and sells 0.5mil. 

4.2 +2 +2 +0.5  = 8.7ml. Crazy number. 

Yes I predicted ~8 million

Anything is possible when you're competing against a Wii U and an XB1
P:

I'd go with 6.5-7 million total by March 31st.

Although I think it's possible they air shipped some supply to get more in December, which could reduce available supply in January.

Currently they're not even competing against them. They're coimpeting against slow chinese assembly line workers.

This.

But my prediction is 7.2m.

Its not impossible but somewhat unlikely.

They can't use something not manufactured yet...

They are producing about 200-250k units per week (~1.2m units per month)... so it is not like they can use the January production... they used in December what they produced until Dec 20th (they need ~1.5 week to ship to US).

The units US ar3 getting in early January are produced in late December... and the units produced this week will reach the stores in one or two weeks.

So what they produced from late Dec to mid January will get into stores in January yet.

The same for February and March... Sony won't stop to ship PS4 to stores.

1million sales for Japan day one? Highly doubt that.
I think 7million by end of March will be accomplished.