How to change number of items on add new pluggins page from backend?

You can filter the parameters for this view per install_plugins_table_api_args_search. See wp-admin/includes/class-wp-plugin-install-list-table.php for the details.

You get an array as argument that looks like this:

Array
(
    [page] => 1
    [per_page] => 30
    [fields] => Array
        (
            [last_updated] => 1
            [icons] => 1
            [active_installs] => 1
        )

    [locale] => en_US
    [installed_plugins] => Array
        (
            [0] => magic-widgets
            [1] => query-monitor
        )

    [search] => seo
)

So you can just set the per_page entry to a higher value. This will also slow down the response, because more data is transferred from wordpress.org.

Here is a sample plugin:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: 200 plugins per install search
 */

add_filter( 'install_plugins_table_api_args_search', function ( array $args ) {

    $args['per_page'] = 200;
    return $args;
});