Checking if all players died?

Well, using your example it's actually easier than you think:

game.Players.PlayerAdded:connect(function(player)
    t=Instance.new("BoolValue", player)
    t.Name="Died"
    player.CharacterAdded:connect(function()
        player.Character.Humanoid.Died:connect(function()
            player.Died.Value=true
            trol=0
            for i,v in pairs (game.Players:GetChildren()) do
                if v.Died==true then
                    trol=trol+1
                end
            end
            num=game.Players:GetChildren()
            if trol=<#num then
                --do stuff...
                for i,v in pairs(game.Players:GetChildren())do
                    v.Died.Value=false
                end
            end
        end)
    end)
end)

There's a direct bool example, or you can use a good o'll table to do the same type of thing:

ppl={}
game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function()
        player.Character.Humanoid.Died:connect(function()
            if not ppl[player.Name] then
                ppl[player.Name]=true
            end
            num=game.Players:GetChildren()
            if  #ppl>=#num then
                --do stuff
                ppl={}
            end
        end)
    end)
end)

Try it out but I won't guarantee it will work XD

There are multiple way you may see which players are alive, like if you made a game and wanted to see if a player died in that round or not, if so, reward, if not, don't reward. A good way is to make a table at the start and on the death of the player, remove their name.

local AlivePlrs = {}
game.Workspace.ChildAdded:connect(function(Char)
    if Char:IsA("Model") and Char:FindFirstChild("Humanoid") then -- Make sure it's a Character, not something else
        local Humanoid = Char.Humanoid
        table.insert(AlivePlrs, #AlivePlrs + 1, Char.Name)
        Humanoid.Died:connect(function() 
            for Position, Plr in ipairs(AlivePlrs) do
                if Plr == Char.Name then -- If they are in the table then...
                    table.remove(AlivePlrs, Position) -- Position is an iterator which gives a number, in this case we can use that number to find where their name is in this table and remove it
                end
            end
        end)
    end
end)

This is a Script btw. The Script I gave above is the basic Idea, of course this would allow a new player who joined the game during a round to be assigned to being Alive, giving them a reward when you probably wouldn't want them to count. I just gave the basic idea, not the whole framework of a game and it's Logic.