Need help in limiting no of process

Hello and welcome,

You do not specify how You add those actions.

You need to keep the state of the pending list.

If You add actions, but the pending list is full, reject with an error.

Please show your user GenServer, it might be the place to hold the pending list.

Tasks.ex

defmodule Tasks do
  def create_user(user) do
    Tasks.DynamicSupervisor.create_user(user)
  end

  def add_tasks(user, task) do
    Tasks.Worker.create_a_task_for_user(user, task)
  end
end

DynamicSupervisor

defmodule Tasks.DynamicSupervisor do
  use DynamicSupervisor

  def start_link() do
    DynamicSupervisor.start_link(__MODULE__, [], name: __MODULE__)
  end

  def create_user(user) do
    case DynamicSupervisor.start_child(Tasks.DynamicSupervisor, %{
           id: user,
           start: {Tasks.Worker, :start_link, [String.to_atom(user)]}
         }) do
      {:ok, _} -> {:ok}
      {:error, {:already_started, _}} -> :user_already_exists
    end
  end

  def init(_args) do
    DynamicSupervisor.init(strategy: :one_for_one)
  end
end

Worker

defmodule Tasks.Worker do
  use GenServer

  @worker __MODULE__

  def create_a_task_for_user(user, _task) do
    get_no_of_running_process_for_a_user(user)
    |> IO.inspect()
  end

  defp get_no_of_running_process_for_a_user(user) do
    {:status, pid, _, _} = :sys.get_status(String.to_atom(user))

    pid
    |> Process.info()
    |> Keyword.fetch!(:message_queue_len)
    |> Kernel.>=(10)
    |> IO.inspect()
  end

  def start_link(user) do
    GenServer.start_link(@worker, user, name: user)
  end

  def init(user) do
    {:ok, user}
  end
end

Here is the code