Use the custom archive setting to modify the number of posts on a custom post type archive page.

sample.php

<?php
/**
 * Use the custom archive setting to modify the number of posts on a custom post type archive page.
 */
 
 add_action( 'pre_get_posts', 'cpt_as_pre_get_posts' );
/**
 * Callback for WordPress 'pre_get_posts' action.
 *
 * Modify the number of posts on an testimony archive page if set to a custom value.
 *
 * @see https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
 * 
 * @since 0.0.1
 * 
 * @return void
 */
function cpt_as_pre_get_posts( $query ) {

    //* if admin or not on the testimony archive page, exit
    if ( is_admin() || ! is_post_type_archive( 'testimony' ) ) {
        return;
    }

    //* get the custom posts_per_page value
    $posts_per_page = (int) genesis_get_cpt_option( 'posts_per_page', 'testimony' );

    //* if a valid posts_per_page value and on the main query, adjust the query
    if ( $posts_per_page && $query->is_main_query() ) {
        $query->set( 'posts_per_page', $posts_per_page );
    }

}