How do I set the number of nodes visible on the first page?

If you are not using the view as the main page, but using Drupal 7, then the number of nodes visible on the first page is set in the admin / config / system / site-information.

If you are using the Front Page view as the first page, in addition to the previous setup for this view, there is a parameter for admin / structure / views / view / frontpage / edit that changes the number of nodes shown on one pager page.