Can you please provide a link to your site, and let us know what theme you’re using? And is it safe to assume your question pertains to WooCommerce?
If so, this article on the official WooCommerce Docs site includes the custom code required to modify the number of products that appear in a row by default:
But that article assumes you’re using a WooCommerce default theme. If your site uses a custom or third-party theme, that code snippet may not work for you.
Some themes include the ability to customize the layout of items on your pages. If your theme does, you’ll likely find that functionality within the Customizer.
And of course, if you find that the only way to customize the appearance of items on your site is to edit the underlying code, you can hire a WooCommerce expert to knock it out for you in just a few minutes.
Hope this at least helps point you in the right direction, having not seen your site.
This page demonstrates how to use the Customizer to change the number of products that show in a single row, or how many rows of products to display on a page:
Hope this helps!