function posts_per_day_by_authorid( $userid, $post_type, $year, $month, $day ) {
      $query = new WP_Query( array(
        'author'         => $userid,
        'post_type'      => $post_type,
        'date_query'     => array(
           array(
             'year'      => $year,
             'month'     => $month,
             'day'       => $day,
             ),
            ),
        'posts_per_page' => -1,
        ));
      $count = $query->post_count;
      return $count;
    }

echo posts_per_day_by_authorid( '1', 'post', '2018', '12', '13' );

<?php

$path = $_SERVER['DOCUMENT_ROOT'];
require_once $path . '/wp-load.php';
global $wpdb;

function posts_per_day_by_authorid( $userid, $post_type, $year, $month, $day ) {
  $query = new WP_Query( array(
    'author'         => $userid,
    'post_type'      => $post_type,
    'date_query'     => array(
       array(
         'year'      => $year,
         'month'     => $month,
         'day'       => $day,
         ),
        ),
    'posts_per_page' => -1,
    ));
  $count = $query->post_count;
  return $count;
}

echo posts_per_day_by_authorid( '1', 'post', '2018', '12', '13' );

?>