In japan there are around +620.000 PS4 solds (July 2014) and aprox. 45.000 X1 (Nov 2014). If FFXV were to be realese on either 2015 or 2016 i woulds predict that there would be around less than 1.000.000 units (PS4+X1), my prediction is that in Japan XV will sell around 500.000 units and if the reviews are good it will rise the PS4(X1?) Sales and rise the solds unit to +1.000.000 .
As for worldwide i predict at the very lest 1.500.000 units sold (for both consoles), i personally would like the game to top in Square's top 5 best selling games