Looking to program/fix my "Ready Up" button?

This is not the proper way to use Remote Events. Click Event connection should be done in the local script, and only after a click a Remote Event should be fired. There are many issues, but the worst is that you make a new connection everytime an event is fired.

You would need to provide your local scripts to get an accurate answer, but I will try my best to do an educated guess:

First create new event in replicated storage called UpdateCountEvent

Next your local scripts. Make a new one, if you do not have it, and parent it to the gui button: Ready button (debounce is not needed in click events):

local button = script.Parent
local RemoteEvent = game.ReplicatedStorage:WaitForChild("RemoteEvent")

button.MouseButton1Click:Connect(function()
	RemoteEvent:FireServer()	
end)

BackButton:

local button = script.Parent
local RemoteEvent2 = game.ReplicatedStorage:WaitForChild("RemoteEvent2")

button.MouseButton1Click:Connect(function()
	RemoteEvent2:FireServer()
end)

Following local script should be parented to the text label

local TL = script.Parent
local UpdateCountEvent = game.ReplicatedStorage:WaitForChild("UpdateCountEvent")

UpdateCountEvent.OnClientEvent:Connect(function(n)
	TL.Text = n .. "/8 Players"
)

Now the server script

local RemoteEvent = game.ReplicatedStorage.RemoteEvent
local RemoteEvent2 = game.ReplicatedStorage.RemoteEvent2
local UpdateCountEvent = game.ReplicatedStorage.UpdateCountEvent

local n = 0
local participatingPlayers = {}
RemoteEvent.OnServerEvent:Connect(function(player)
	
	if participatingPlayers[player] then
 		return
	end
	n = n + 1
	participatingPlayers[player] = true
	UpdateCountEvent:FireAllClients(n)

end)
RemoteEvent2.OnServerEvent:Connect(function(player)
	
	if not participatingPlayers[player] then
 		return
	end
	n = n - 1
	participatingPlayers[player] = nil
	UpdateCountEvent:FireAllClients(n)

end)

--Edit (see below)
local CheckPlayerCount = game.ReplicatedStorage.CheckPlayerCount

local function OnCheckPlayerCount()
	return n
end
CheckPlayerCount.OnServerInvoke = OnCheckPlayerCount

--Edit2:
game.Players.PlayerRemoving:Connect(function(player)
	if participatingPlayers[player] then
		participatingPlayers[player] = nil
		n = n -1
		UpdateCountEvent:FireAllClients(n)
	end
end)

I gtg now, but I will be back later in case of problems.

Edit: You'll still need to let joining players know how many participants are Ready Up. Lazy way would be to fire UpdateCountEvent inside a function connected to PlayerAdded Event. However you may risk, that the gui has not loaded yet. Some people add wait, before firing, but I prefer more elegant solution: Add a RemoteFunction in ReplicatedStorage and name it CheckPlayerCount Add to your server script this lines (I also edited answer above):

local CheckPlayerCount = game.ReplicatedStorage.CheckPlayerCount

local function OnCheckPlayerCount()
	return n
end
CheckPlayerCount.OnServerInvoke = OnCheckPlayerCount

Finally modify update script like this:

local TL = script.Parent
local UpdateCountEvent = game.ReplicatedStorage:WaitForChild("UpdateCountEvent")
local CheckPlayerCount = game.ReplicatedStorage:WaitForChild("CheckPlayerCount")

TL.Text = CheckPlayerCount:InvokeServer() .. "/8 Players"

UpdateCountEvent.OnClientEvent:Connect(function(n)
	TL.Text = n .. "/8 Players"
)

Edit 2: To complete your script, it is a good idea to remove players that have quit the game, being impolite and not pressing back button before leave: Add few more lines to your local script (I edited the answer above too):

game.Players.PlayerRemoving:Connect(function(player)
	if participatingPlayers[player] then
		participatingPlayers[player] = nil
		n = n -1
		UpdateCountEvent:FireAllClients(n)
	end
end)