Depends on many things: Core speed Network upload speed How spread will the players be? Do you have big worlds and the players a widely spread or do you have small worlds, and many players are on one single spot. Please answer these questions/provide these facts.
The main influencer in your pocketmine.yml is your chunk sending, but default is actually really good at 50 players, I find. Try using /timings to see what's causing your lag, it's a really helpful tool if you understand it.
Not really. It depends on far more than that...I'd like him to send us the info i requested above, to make a perfect pocketmine.yml. Maybe I should even make some sort of pocketmine.yml optimiser where you enter your core speed etc. and get a pocketmine.yml optimised for your setup.
Or... Someone could explain here how to tweak the parameters for best performance depending on hardware/network/server type. The infamous mcg76 wrote an excellent article on the subject which unfortunately seems to have disappeared. I'm no expert but I have had good results increasing compression for example, when CPU is not a problem, and other things I might elaborate on if I get the time.