You can increase or decrease the number of related products that appear on the product details pages for your products in your woocommerce catalog.

Ideally you would want to use the functions.php file inside of a child theme for this or create one in your child theme to use if one does not exist. You can also use the functions.php inside your parent theme directory. Remember you may lose the change when your theme updates if you use your parent functions.php.

Open the functions.php file you wish to modify with your text/code editor of choice.

At the very end of your functions.php, so that this is the last bit of code on your file, place the following code:

// Adjust woocommerce_output_related_products() 
function woocommerce_output_related_products() { 
woocommerce_related_products(4,2); // Will display 4 products in 2 rows 
}

In line 3 in the code above you will see (4,2). The first number (4) is how many products you want to show and the second number (2) is the number of rows to organize the products.

In Detial

The default number and rows in woocommerce is two products, in two rows which looks like:

// Adjust woocommerce_output_related_products() 
function woocommerce_output_related_products() { 
woocommerce_related_products(2,2); // Will display 2 products in 2 rows 
}

This code will display related products as:

number of related products in woocommerce

To add two more products to the related products section you can change the code to:

// Adjust woocommerce_output_related_products() 
function woocommerce_output_related_products() { 
woocommerce_related_products(4,2); // Will display 4 products in 2 rows 
}

Which increases the number of products and organizes the products like so:

number of related products in woocommerce

You can change these numbers to as many or as few products and rows as you would like to fit your needs.