Related Products:

Copy/paste vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml to  app/design/frontend/Vendor/default/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 ...

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 ...