"Tonumber" keeps on giving a nil value?

To get your main question out of the way:

You're trying to using tonumber() on a table. This won't return a number since you're using it on a table. To get the length of a table you use the '#' operator before table returning the length of it. You use tonumber() to convert another data type to a number type.

local Players = game.Players:GetPlayers()
local PlayerAmount = #Players

print(PlayerAmount) --Would print the length of the 'Players' table

Now you have a lot of things in your code that you shouldn't do. One of those things is how you're defining services and using WaitForChild(). You should only use WaitForChild() when getting things like objects inside of services (I usually use it in client side of things)

When getting a service use the :GetService() method.

local RepStorage = game:GetService("ReplicatedStorage")
local Players = game:GetService("Players")

--This is an ok use of WaitForChild
local Remote = RepStorage:WaitForChild("GameRemote")

Second, you should use functions to run code that you know you'll be using later.

--Demonstration of a wait for players function:

local function CheckPlayerCount()
	local Players = game.Players:GetPlayers()
	local PlayerAmount = #Players

	return PlayerAmount
end

while true do
	local playersActive = CheckPlayerCount()

	if playersActive > 2 then
		Remote:FireAllClients()
	end

	wait(5) --intermission for a generic game loop
end


--Down here would be the rest of your OnServerEvent code.

edit: fixed the rest of your code