How do I detect the number of players in a certain area?

EDIT

im going to use magitude to get the amount of players in a radius of 50 studs every 30 seconds

i inserted a part into workspace and placed this script inside it

while true do
local list = game.Players:GetChildren() --makes a list of the current players
local counter = 0
local distance = 50 ----as in 50 studs
for i= 1, #list do 

    local plyrpos = list[i].Character:WaitForChild("HumanoidRootPart").Position
    local centerpos = script.Parent.Position

    local testdistance = (plyrpos-centerpos).magnitude--- get magnitude
        if testdistance<=distance then
            counter= counter+1 ----if they are in the region then add to counter
        end

end
print("there are "..counter.." players inside")
count=0
wait(30)
end

here (the green part is 50 studs long). when 3 players are inside 50 stud radius it prints 3 players

here is a test with a single player inside the radius

here the blue sphere is the actual depiction of the region.