How can i retrieve default post per page value? from settings->reading. And total number of posts?

It’s saved in an option:

$default_posts_per_page = get_option( 'posts_per_page' );

Reference: get_option Parameters

You can see the keys and values of all available options by manually entering the admin url:

/wp-admin/options.php

Edit

Total number of posts:

global $wp_query;
$total_posts = $wp_query->post_count;