And they might offer their economy-passengers a bit more comfort than on the B777-300ER with HD cabin. The 3-4-3 seating in a 300ER is something Amnesty International has on its watch. 😀
Applied the same or a slight higher level of humanity (3-4-3 is OK in A380), the A380-800 is good for ~600 to 650 Paxe. Singapore has very low density has very low density First and business (1-2-1 for both).

Applying the same level of humanity requires 3-5-3 on A380, seeing that A380 is 20 cm wider than 747 and 46 cm wider than 777.