Elle here from Shopify.
Most themes will give you the option to modify the way your blog appears within the theme editor under Online store > Themes > Customize > Select 'Blog' from the dropdown menu. Once you select 'blog' from the dropdown menu, you can select the option on the left hand side within the editor to modify how the blog page appears. We have a helpful guide on customizing themes here.
If there is no option to change how many columns per page within the editor, then you will need to modify the code of your theme. If you need coding assistance, please post your store's URL along with the name of the theme you're using, and members of our community will be able to assist further! Alternatively, our Shopify Experts are available for small jobs like this one!
Hope this helps!