how do i make a countdown the same for everyone using a remote event?

ANSWER: You need the counter to be on the server. And you need to pass the counter's value through RemoteEvent to the other clients, and not the other way around.

Server Script (Normal Script in the serverscriptservice):

local RemoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")

local count = 0

while wait(1) do
	count += 1
	print("SERVER COUNT:" .. count)
	RemoteEvent:FireAllClients(count)
end

Client Script (Local Script in the starterplayerscripts):

local RemoteEvent = game:GetService("ReplicatedStorage"):WaitForChild("RemoteEvent")

local count

RemoteEvent.OnClientEvent:Connect(function(c)
	count = c
	print("CLIENT COUNT:" .. count)
end)