Is there a problem with text output for all players?

The problem is with the conditional statement.

Let's take a closer look with an example:

local tableOfPlayers = { -- pretend these are players and not strings
	"Player1",
	"Player2",
	"Player4" -- player 3 had sick day
}

for index, player in pairs(tableOfPlayers) do
	print(index, player)
end

-- OUTPUT:
1, Player1
2, Player2
3, Player4

Index, or in your case i, is simply the place in the table at which the value is indexed. So, if we add a conditional like in your case:

local tableOfPlayers = { -- pretend these are players and not strings
	"Player1",
	"Player2",
	"Player4" -- player 3 had sick day
}

local neededPlayers = 2

for index, player in pairs(tableOfPlayers) do
	if i < neededPlayers then
		print("Not enough players")
	else
		print(index, player)
	end
end

-- OUTPUT:
Not enough players
Not enough players
3, Player4

See? Even though there are enough players, it doesn't check if there are, only if the index of some players is higher than needed to start. To fix this, we change the conditional.

Fixed code

Here's an example of how you could be able to fix your code:

local players = game:GetService("Players")
local needtostart = 2

players.PlayerAdded:Connect(function(plr)
	if #players:GetPlayers()>=needtostart then
		for _, plr in pairs(players:GetPlayers()) do
			plr:WaitForChild("PlayerGui"):WaitForChild("TopGui").MainFrame.ReadyInfo.Text = "Enough players"
		end
	else
		for _, plr in pairs(players:GetPlayers()) do
			plr:WaitForChild("PlayerGui"):WaitForChild("TopGui").MainFrame.ReadyInfo.Text = "Not enough players"
		end
	end
end)

Don't use this yet.

But, if FilteringEnabled is on like it should be, the server can't see GUIs in PlayerGui which it did not create. So, you'll either need to use remote events or clone the GUIs into PlayerGui manually. I suggest Remotes. I just used a local script though.

Actually correct example:

You can just use a LocalScript in the end.

-- Local Script in PlayerGui
local plr = game.Players.LocalPlayer
local gui = plr:WaitForChild("PlayerGui"):WaitForChild("TopGui").MainFrame.ReadyInfo

local neededPlayers = 2

local function update()
	if(game.Players.NumPlayers >= neededPlayers)then
		gui.Text = "Enough players."
	else
		gui.Text = "Not enough players."
	end
end
update() -- update when they join

game.Players.PlayerAdded:Connect(function()
	update() -- update when new player joins
end)

game.Players.PlayerRemoving:Connect(function()
	update() -- update when player leaves
end)