Google. Some employment websites, some sourced from Wikipedia citations, I did have some standards and didn't take LinkedIn headcounts or other dubious sources. Sadly, Sony's rather generic studio names (and the weird status of stuff like XDev and PSS VA) make them harder to Google and thus I couldn't find reliable numbers for some like San Diego.
Well, XDEV isn't a studio. It started as a division within Liverpool Studio that branched out and become its own studio to manage 'external development' (this is the origin of the XDEV name), outsourcing made in other studios for Liverpool and nearby Sony studios, plus also the management and publishing of 2nd party games. They also added later there PR/CM/CS support for some European 1st and 2nd party games, but also from other locations.
They did grow and basically became in charge of managing 2nd party publishing and outsourcing for Europe. Then in USA they also had people doing 2nd party publishing and outsourcing for American teams inside some internal development studios, mostly Sony Santa Monica. They also did branch out outside these American internal development studios to become an external team to them, which later became the American branch of XDEV.
And later same happened in Japan: the 2nd party publishing and outsourcing for the Japanese teams was handled by Japan Studio, but in the recent restructuring where it was rebranded to Team Asobi, their 2nd party publishing and outsourcing team was branched out to become a different team/studio, which now would be the Asian branch of XDEV to manage the 2nd party publishing and outsourcing for the Asian (now not only Japanese teams).
I mean, XDEV (or their 'External Development' team) isn't a development studio itself. Now it's a global team with people/ regional teams in several locations across the world.
Then there's another global team: Sony PlayStation Visual Arts Service Group (VASG), sometimes also mentioned as Visual Arts, Creative Arts or Product Development Services Group. They have offices in San Diego (the original one) or more recently also Malaysia (PlayStation Studios Malaysia) and are an internal support team to help the games developed by the other PS Studios teams like ND, SSM, Guerrilla, Japan Studios, Insomniac etc (and sometimes even games from outside Sony) in areas like motion capture, face/body scanning, art and animation, pretty often for cinematics.
Since they only do outsourcing/support job and never have been lead studio of any game -VASG started doing it for the TLOU remake, a project pitched by them, but later ND became the lead of this project because the higher ups weren't happy with the results-, they often aren't counted as PS Studios development teams.