This is how you change the number of columns for similar products.

You can simply change the number of products that are displayed via a plugin or with PHP.

Since only a small code snippet is needed for the change, it is recommended to implement the whole thing with PHP.

In the Functions.php of your Child Theme, you can insert the following code to influence the number and columns:

/**
 * Change number of related products on product page
 */
function related_products_columns( $args ) {
   $args['posts_per_page'] = 4; // 4 related products
   $args['columns']        = 4; // arranged in 4 columns

   return $args;
}

add_filter( 'woocommerce_output_related_products_args', 'related_products_columns' );

That was it. Now you can customize your selection according to your wishes.