max_users attiribute does not work

You need to differentiate:

• The option max_users for MUC service is by default 200.
• The option max_users for a newly created room is by default 200, too.

Let's imagine that you want the MUC service to allow as much as 1000 users, and you also want new rooms to allow by default 500 max.
Try this configuration:

{modules, [
  {mod_muc, [
    {max_users, 1000},
    {default_room_options, [
      {max_users, 500}
    ]},
    {access, muc},
    {access_create, muc},
    {access_persistent, muc},
    {access_admin, muc_admin},
    {history_size, 1}
  ]},
  ...
]}.

The owner of a room can modify the max_users of that room using a Jabber client, so instead of 500 he can put: 5, 10, 20, 30, 50, 100, 200, 500, 1000.
But he can't put more than 1000 because you put that limit in mod_muc.