TaskList of GitHub Flavored Markdown doesnt support ordered lists

No, it's not supported. You can write an ordered list of checkboxes like this:

1. [ ] Foo
2. [ ] Bar

It renders as an ordered list in HTML (<ol>), but their stylesheet removes the numbers.

So while you can express the list as an ordered list, you would need your own stylesheet to render it as such.

Note: since the introduction of nested task list (May 2014), numbered task list are still not supported, even though the following would still be rendered with <ol> elements:

1. [ ] Figure out wormholes
  1. [ ] Call @arfon
  2. [ ] Research ([docs](http://en.wikipedia.org/wiki/Wormhole#Time_travel))
  3. [ ] Build prototype #15
  4. [ ] Test run #43 @world-domination/time-travel
2. [ ] ...?
3. [ ] Profit!

And that would give:

As in:

<ol class="task-list added">

    <li class="task-list-item">
        <input class="task-list-item-checkbox" type="checkbox" disabled=""></input>
        <label></label>

         Figure out wormholes 

        <ol class="task-list">
            <li class="task-list-item"></li>
            <li class="task-list-item"></li>
            <li class="task-list-item"></li>
            <li class="task-list-item"></li>
        </ol>
    </li>
    <li class="task-list-item"></li>
    <li class="task-list-item"></li>

</ol>