Check if post with same meta value exists

Try below code.

$args = array(
   'fields' => 'ids',
   'post_type'   => 'numbers',
   'meta_query'  => array(
     array(
     'key' => 'number',
     'value' => $number
     )
   )
 );
 $my_query = new WP_Query( $args );
 if( empty($my_query->have_posts()) ) {
   $post_id = wp_insert_post(array (
       'post_type' => 'numbers',
       'post_title' => $name,
       'post_content' => $details,
       'post_status' => 'draft',
       'tax_input' => $custom_tax,
   ));
   if ($post_id) {
       // insert post meta
       add_post_meta($post_id, 'number', $number);
   }
 }

If there is same number in postmeta it will not allow to create post and post meta.

Hope this will helps you.