Trying to send AJAX data to WordPress hook

by

Untested but …

You need to pass your “action” identifier with the data, like so:

var data = {
    'action': 'my_action',
    'whatever': 1234
};

In your case, that action would be get_number_of_promos.

The example is from the Codex page about AJAX, which you should read carefully: https://codex.wordpress.org/AJAX_in_Plugins