Programatically create multiple posts for the given post type. This function runs on every page reload so be sure to run it only once

functions.php

function programmatically_create_multiple_posts() {

	// Initialize the page ID to -1. This indicates no action has been taken.
	$post_id = -1;
	$numberofposts = 40; 
	$new_post_type = 'portfolio';

for ($i=0; $i < $numberofposts; $i++) { 	
// Setup the author, slug, and title for the post


	$author_id = 1;
	$slug = 'example-post '. $i;
	$title = $new_post_type . ' dummy '. $i;

	// If the page doesn't already exist, then create it
	if( null == get_page_by_title( $title ) ) {

		// Set the post ID so that we know the post was created successfully
		$post_id = wp_insert_post(
			array(
				'comment_status'	=>	'closed',
				'ping_status'		=>	'closed',
				'post_author'		=>	$author_id,
				'post_name'			=>	$slug,
				'post_title'		=>	$title,
				'post_status'		=>	'publish',
				'post_type'			=>	$new_post_type,
			)
		);

	// Otherwise, we'll stop
	} else {

    		// Arbitrarily use -2 to indicate that the page with the title already exists
    		$post_id = -2;

	} // end if
}	

} // end programmatically_create_post
add_filter( 'template_redirect', 'programmatically_create_multiple_posts' );