Change number of posts on consecutive category pages (Wordpress)

Question

I am trying change the number of posts that are displayed on a category pages to change on consecutive pages (page 2, 3, etc). So page one displays 7 posts, but pages 2, 3 and 4

Answer 1

This is from an answer I recently done on WPSE. I have made some changes to suite your needs. You can go and check out that post here

STEP 1

If you have changed the main query for a custom query, change it back to the default loop

<?php

        if ( have_posts() ) :
            // Start the Loop.
            while ( have_posts() ) : the_post();

                ///<---YOUR LOOP--->

            endwhile;

                //<---YOUR PAGINATION--->   

            else : 

                //NO POSTS FOUND OR SOMETHING   

            endif; 

    ?>

STEP 2

Use pre_get_posts to alter the main query to change the posts_per_page parameter on the category pages

STEP 3

Now, get the posts_per_page option set from the back end (which I assume is 6) and also set your offset which we are going to use. That will be 1 as you will need 7 posts on page one and 6 on the rest

$ppg = get_option('posts_per_page');
$offset = 1;

STEP 4

On page one, you'll need to add the offset to posts_per_page will add up to 7 to get your seven posts on page one.

$query->set('posts_per_page', $offset + $ppp);

STEP 5

You must apply your offset to all subsequent pages, otherwise you will get a repetition of the last post of the page on the next page

$offset = $offset + ( ($query->query_vars['paged']-1) * $ppp );
$query->set('posts_per_page',$ppp);
$query->set('offset',$offset); 

STEP 6

Lastly, you need to subtract your offset from found_posts otherwise your pagination on the last page will be wrong and give you a 404 error as the last post will be missing due to the incorrect post count

function category_offset_pagination( $found_posts, $query ) {
    $offset = 1;

    if( !is_admin() && $query->is_category() && $query->is_main_query() ) {
        $found_posts = $found_posts - $offset;
    }
    return $found_posts;
}
add_filter( 'found_posts', 'category_offset_pagination', 10, 2 );

ALL TOGETHER

This is how your complete query will look like that should go into functions.php

function ppp_and_offset_category_page( $query ) {
  if ($query->is_category() && $query->is_main_query() && !is_admin()) {
    $ppp = get_option('posts_per_page');
    $offset = 1;
    if (!$query->is_paged()) {
      $query->set('posts_per_page',$offset + $ppp);
    } else {
      $offset = $offset + ( ($query->query_vars['paged']-1) * $ppp );
      $query->set('posts_per_page',$ppp);
      $query->set('offset',$offset);
    }
  }
}
add_action('pre_get_posts','ppp_and_offset_category_page');

function category_offset_pagination( $found_posts, $query ) {
    $offset = 1;

    if( !is_admin() && $query->is_category() && $query->is_main_query() ) {
        $found_posts = $found_posts - $offset;
    }
    return $found_posts;
}
add_filter( 'found_posts', 'category_offset_pagination', 10, 2 );