How to make sure all my npc's have died?

So, this question arose by curiosity from another person's question I was trying to answer. I have a lot of enemy npcs in a folder, and i want a gate to open when all of them have died. Here is what I have so far, Any help is appreciated!

local part1 = game.Workspace.Part1
local part2 = game.Workspace.Part2
local t = 0
local npcf = game.Workspace.NPCF:GetChildren() -- npcf is the folder and I am getting all the npcs using GetChildren()

local TweenService = game:GetService("TweenService") -- these are the tweens which should play if it has died.

local part1 = game.Workspace.Part1

local goal = {}
goal.Position = Vector3.new(-6.197, 22.353, -67.553)

local tweenInfo = TweenInfo.new(5)

local tween1 = TweenService:Create(part1, tweenInfo, goal)

local TweenService = game:GetService("TweenService")

local part2 = game.Workspace.Part2

local goal = {}
goal.Position = Vector3.new(-23.852, 21.838, -66.892)

local tweenInfo = TweenInfo.new(5)

local tween2 = TweenService:Create(part2, tweenInfo, goal)

local npc2 = game.Workspace.NPC2


while true do
	for i = 1,#npcf do -- I have no clue what to do for multiple npcs and how to constantly check if ALL OF THEM HAVE DIED
		local npcnum = npcf[i]
		if npcnum.Humanoid.Health == 0 then
			print("npc"..i.."died")
		end
	end
	if npc2.Humanoid.Health == 0 then -- this one works, but its for only1 npc cuz it only checks constantly if 1 npc has died
		tween2:play()
	end
	wait()
end

Instead of using

for i = x, y do

end

use

for i, v in pairs() do

end

as shown below:

local part1 = game.Workspace.Part1
local part2 = game.Workspace.Part2
local t = 0
local npcf = game.Workspace.NPCF:GetChildren()

local TweenService = game:GetService("TweenService")

local part1 = game.Workspace.Part1

local goal = {}
goal.Position = Vector3.new(-6.197, 22.353, -67.553)

local tweenInfo = TweenInfo.new(5)

local tween1 = TweenService:Create(part1, tweenInfo, goal)

local TweenService = game:GetService("TweenService")

local part2 = game.Workspace.Part2

local goal = {}
goal.Position = Vector3.new(-23.852, 21.838, -66.892)

local tweenInfo = TweenInfo.new(5)

local tween2 = TweenService:Create(part2, tweenInfo, goal)

local npc2 = game.Workspace.NPC2


while true do
    for i, npc in pairs(npcf) do -- Just loops through the table contents
        if npc.Humanoid.Health == 0 then
            print("npc"..i.."died")
        end
    end
    if npc2.Humanoid.Health == 0 then
        tween2:play()
    end
    wait()
end