Generating User(s) with Settings API

You are using add_settings_field wrong: the function sl_wp_create_user is being passed as $args (and you probably don’t need any args), and it should be the sanitize callback for register_settings.

Make the input like:

function sl_render_users_input() {
    echo '<input id="num_users[value]" name="num_users[value]" type="input" value=""  />';
}

And the sanitize function:

function sl_wp_create_user( $input ) {
    if( isset( $input['value'] ) ) {
        // do your thing
    }
}