SERVER SCRIPT:

local insert = game:GetService("InsertService")

game.ReplicatedStorage.RemoteEvent.OnServerEvent:Connect(function(Player, ID)
	if tonumber(ID) == nil then return end
	print(ID)
	local item = insert:LoadAsset(ID)
	local Child = item:GetChildren()[1]
	if Child:IsA("Decal") then
		Player.Character.Head.face.Texture = Child.Texture
	elseif Child:IsA("Accessory") then
		Player.Character.Humanoid:AddAccessory(Child)
	elseif Child:IsA("Shirt") then
		if Player.Character:FindFirstChildWhichIsA("Shirt") then
			Player.Character.Shirt.ShirtTemplate  = Child.ShirtTemplate
		else
			local Shirt = Instance.new("Shirt", Player.Character)
			Shirt.Name = "Shirt"
			Shirt.ShirtTemplate = Child.ShirtTemplate
		end
	elseif Child:IsA("Pants") then
		if Player.Character:FindFirstChildWhichIsA("Pants") then
			Player.Character.Pants.PantsTemplate = Child.PantsTemplate
		else
			local Pants = Instance.new("Pants", Player.Character)
			Pants.Name = "Pants"
			Pants.PantsTemplate = Child.PantsTemplate
		end
	end
end)

LOCAL SCRIPT:

script.Parent.TextButton.MouseButton1Click:Connect(function()
	for _, v in pairs(string.split(script.Parent.TextBox.Text, ",")) do
	game.ReplicatedStorage.RemoteEvent:FireServer(v)
	end
end)

Whenever I put more than 1 ID, it errors:

13:16:50.728 - Unable to cast string to int64
13:16:50.738 - Stack Begin
13:16:50.739 - Script 'Workspace.Script', Line 6
13:16:50.739 - Stack End
Anonymous