What can I add to the script so the system only sends a minimum of 1 person for each teleport block?

For the teleporting players, you're basically passing a table as an argument to the function. Here's how I'd set it up:

local targets = {} --Either a table or a model of the parts you want to teleport players to. If it's a model, then define it as model:GetChildren()

local function teleport(targets)

local targets = targets --Defines targets as local so that manipulating the table in the loop doesn't effect the one outside the function

for i,v in ipairs(game.Players:GetChildren()) do --ipairs is a lot like pairs, except it stops iterating if it finds a nil value in the table

local ran = math.random(1,#targets) --returns a random number bewteen 1 and the length of the table

pcall(function() v.Character.HumanoidRootPart.CFrame = targets[ran].CFrame end) --pcall runs a function in safemode, so if it results in an error, it keeps running the following code. It also returns a bool and an error message. I used an anonymous function in it, so that I don't have to redefine one every time we call the function in the loop.

table.remove(targets,ran)--removes the target from targets to prevent players from being teleported to the same target

end--end loop
end--end function

for i = 20,0,-1 do --countdown to give players time to join, optional.
wait(1)
print(i)
end

teleport(targets)--calls the function