How do I make a script only run when a team has no players?

Insert a number value into the workspace. Set it at 0 to start off with and name it "TeamPlrz" (Or whatever else you want). Insert a script to do whatever you want and put this:

if game.Workspace.TeamPlrz.Value==0 then
--Put the rest of script here--
end

Now put this LOCALscript into the PlayerGui.

local Plr=game.Players.LocalPlayer

function Change(Color)
    if Color=="" then --Insert the team color that you want to be empty to start the script.
    game.Workspace.TeamPlrz.Value=game.Workspace.TeamPlrz.Value+1
    elseif Color=="" then --Insert the team color that you want players to be in when the script starts.
    game.Workspace.TeamPlrz.Value=game.Workspace.TeamPlrz.Value-1
end

function Left(Player)
    if Player.TeamColor=="" then --Insert the team color that you want to be empty to start the script.
    game.Workspace.TeamPlrz.Value=game.Workspace.TeamPlrz.Value-1
end

game.Players.PlayerRemoving:connect(Left)
Plr.Changed:connect(Change)

Now if you've done this correctly, then it should work. Comment if there's any problems.