Related Products:
Copy/paste
vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml
to
app/design/frontend/Vendor/Theme_name/Magento_Catalog/templates/product/list/items.phtml
In case 'related' block, Change $limit. (ie: $limit = 0 into $limit = 6)
And Read this link for more information:-
Magento2 : Set Product limit in Related and Upsell Product
Hope this help you
Thanks ...