Around 350 was their entire head count, last I check, about 3 months ago. An they specifically stated to shareholders last year they weren't going to go on a hiring spree. Lying about those sort of things is a big no no.
Take away 100 people which are working on Elite Dangerous (confirm by devs and people who visited the studio), let say around 40 of them cover HR, social media and community management, licencing, merchandise, online maintenance, facility manager, IT team.
That leave 210 people, to work one PC dlc, JWE expansions dlc, JWE 2, Planet Coaster 2, Project Marconi, Jurassic World Survival.
Taking that into account, 50 people working on this might be a higher number than the real figure.
But there more.
Frontier has a dedicated team of people working on just improving the cobra engine and developing new in house technologies. So that even less people to work on PZ.
Now if it true JWS is in the work, an I believe it is, to be competitive in today's world of survival games (there a lot of them), that got to have second largest team at frontier already and could easily be their first game to cross a hundred million in sales, so they are going to be throwing a lot of resources at it. In fact I would bet that JW franchise license games, which is now by far Frontier biggest money spinner has the most resources devoted to it. At least a 100 people.
That leave roughly 100 people working on everything else going on at Frontier. So even 50 being devoted to work on PZ is on the optimistic side.
Now explain your reasoning on how you figure there more than 50 people working on the game?