They definitely need more studios. Just because they get more studios doesn't mean the quality of the studios they have will decrease. Plus they need more studios if they are going to properly support vr. Most of Sony's heavy hitters will have released a game last year and supposedly this year from ghost of tsushima, spiderman, last of us, ratchet and clank, horizon zero dawn, and god of war. That pretty much just leaves the Days gone studio when it comes to the cinematic third person games. Unless they push back god of war to next year they have pretty much blown nearly their entire first party studios loads for this gen at pretty much the start of the gen.
and yeah, I know the studios have grown organically lately but that doesn't mean that they aren't using this growth to make the games even larger than before rather than making games quicker.