The posts per page setting is controlled by your theme in the package.json file. The default is 5 posts per page, but you can edit it to anything you require.

How to edit posts per page in Ghost

If you would like to change the number of posts that appear on a page on your publication, update your package.json config file using posts_per_page:

Example:

"config": {    "posts_per_page": 7}

For more information about the package.json file, visit the config guide.