Source

File: wp-includes/user.php

function count_many_users_posts( $users, $post_type = 'post', $public_only = false ) {
	global $wpdb;

	$count = array();
	if ( empty( $users ) || ! is_array( $users ) ) {
		return $count;
	}

	$userlist = implode( ',', array_map( 'absint', $users ) );
	$where    = get_posts_by_author_sql( $post_type, true, null, $public_only );

	$result = $wpdb->get_results( "SELECT post_author, COUNT(*) FROM $wpdb->posts $where AND post_author IN ($userlist) GROUP BY post_author", ARRAY_N );
	foreach ( $result as $row ) {
		$count[ $row[0] ] = $row[1];
	}

	foreach ( $users as $id ) {
		if ( ! isset( $count[ $id ] ) ) {
			$count[ $id ] = 0;
		}
	}

	return $count;
}