How do you check if there is a certaint amount of players?

You could put your main script into a function. You could do that like this:

function Round()
    --Code
end

And I assume you would like for there to be 4 players or higher. So, to do that, you could loop your function and check if there were 4 or more players.

while true do
    if game.Players.NumPlayers >= 4 then
        Round()
    else
        break  --Exits the loop and nothing happens
    end
    wait(1) --Always end your infinite loops (while true do) with a wait. Otherwise, studio will crash and you have to start over.
end

Note: Once you add repeat wait() until game:GetService('Players').NumPlayers >= 4, it pauses the rest of the script until there is 4 or more players.

Run code once if there is 4 or more players:

repeat wait() until game:GetService('Players').NumPlayers >= 4
-- your code here

Run once every time a player joins if there is 4 or more players:

game:GetService('Players').PlayerJoined:connect(function(player)
    if game:GetService('Players').NumPlayers >= 4 then
        -- your code here
    end
end)

Run code every 0.03 seconds if there is 4 or more players:

while true do
    if game:GetService('Players').NumPlayers >= 4 then
        -- your code here
    end
    wait()
end