Cant check number of Players?

You can use a IntValue to keep track of how many players that entered your game. I'll just give you an example and this is just a standard example so better practice can be made later.

In a script:

local Players = game:GetService("Players")
local AmountOfPlayers = nil

Players.PlayerAdded:Connect(function(player)

    -- Check if our game hasn't created the AmountOfPlayers object first
    -- If so create our new object
    if not AmountOfPlayers then
        AmountOfPlayers = Instance.new("IntValue")
        AmountOfPlayers.Name = "AmountOfPlayers"
        AmountOfPlayers.Value = 0
        AmountOfPlayers.Parent = workspace
    end

    -- If our object exists then increment the games player count
    if AmountOfPlayers then
        AmountOfPlayers.Value = AmountOfPlayers.Value + 1
    end
end)

Players.PlayerRemoving:Connect(function(player)
    -- Decrease AmountOfPlayers Value by 1 each time a player is removed from the game.
end)

-- Then you could check in another script like
local playersConnectedAmount = workspace:WaitForChild("AmountOfPlayers")

if playersConnectedAmount.Value == 1 then
    -- Run Code
    ...
end

while wait() do
    if #game:GetService("Players"):GetPlayers() == 1 then
        print("test")
    end
end

If this helped you, please upvote and accept the answer.