Although it's a vgchartz forum I'd rather go with Famitsu numbers when it comes to japanese sales. So going by garaph.info :
There were 4,574,562 PS2s in Japan when FFX launched. It sold 1455k copies on its 1st day, 1,749,737 1st week, and 2,323,463 LTD.
With 3,5m-4m PS3s in Japan by the time FFXIII launches it should be able to have similar numbers.