PlayStation Social Media Manager Sid Shuman revealed the figure on the PS Blog, saying, "Party Voice Chat will support up to 8 players on PS4, like PS Vita." On the subject of Vita, Sony Worldwide Studios boss Shuhei Yoshida tweeted recently that party chat will work between Vita and PS4. So that's cross-platform, cross-game chat - not too shabby, really.
Update: A Microsoft spokesperson told Game Informer the Xbox One also supports up to eight people in party chat. So, final score: Xbox One 8-8 PlayStation 4.