It would be 960 consoles in a 8 hours turn, 2880/day. If we count 20 working days per month, it would be 57600/month. And the year has 12 months, so 691200 consoles/year in that factory.
10 factories like that would make ~7 million PS4s/year, I doubt PS4 will sell more consoles than that at this point of the generation. If you add PS5 and they take the same amount of time probably you'd need another 10 as you mention. If possible, it would be great if these machines can be reprogrammed to switch from PS4 to PS5 since as the generation advances he sales will move from PS4 to PS5, making the total more or less similar.
Maybe some more to include PSVR2 too and in the future PS5 Pro starting at the point where the yearly PS5 sales peak (around 2nd/3rd year).