Is the ability of Sphinx of New Prahv cumulative?
No, just {2}; in this case, Sphinx of New Prahv refers only to itself. (Also, if the card's name somehow changes, it would still refer to itself.) The relevant rule is


  201.4. Text that refers to the object it's on by name means just that particular object and not any other objects with that name, regardless of any name changes caused by game effects.




It would be different ({4}) if the card would read


  Spells your opponents cast that target a card named Sphinx of New Prahv cost {2} more to cast.


but it doesn't.