I have tried this code, but it doesn't work.

function university_adjust_queries($query) {
    $query->set('post_per_page', '1');
}

add_action('pre_get_posts', 'university_adjust_queries');

I have tried this code, but it doesn't work.

function university_adjust_queries($query) {
    $query->set('post_per_page', '1');
}

add_action('pre_get_posts', 'university_adjust_queries');

• query
• pre-get-posts

• You can change how many posts appear on archives in the settings in WP Admin, you don't need a filter for it – Tom J Nowell ♦ Commented Feb 5, 2020 at 21:26

2 Answers 2

You're missing an s.

$query->set('posts_per_page', '1');

However - you probably don't want to do this for every query, since that affects everything - widgets, back end, everything. You should make it conditional. For example:

<?php
function university_adjust_queries($query) {
    // If this is the main query, not in wp-admin, and this is the homepage
    if($query->is_main_query() && !is_admin() && $query->is_home()) {
        $query->set('posts_per_page', '1');
    }
}
add_action('pre_get_posts', 'university_adjust_queries');
?>

This will make sure you're only affecting the main query (The Loop) on the homepage, and only on the front end. You can adjust the condition to target whichever spot you're wanting to affect.

function university_adjust_queries($query) {
  if (!is_admin() AND is_post_type_archive('event') AND $query->is_main_query()) {
        $query->set('posts_per_page', 1);
}