Paging and Order BY

These "variations" you see are the WP Query parameters. In the link you can see all the (possible) combinations that can be made. Answering your particular questions:

  

Show articles with the letter A

Articles that contain the letter A, in any point of the article? Articles whose title starts with the letter A?

Assuming it's the last, I believe you have to do some sort of filter after the query. I do not know if this is only possible using query parameters. You can do something like this:

$args = array(
    'orderby' => 'title',
    'order' => 'ASC',
    'posts_per_page' => -1 //retorna todos os posts da base
);

query_posts($args);

if (have_posts()) { 

    $letra = 'A';

    while (have_posts()){
        the_post();
        $letraInicial = strtoupper(substr($post->post_title,0,1));
        if ($letraInicial == $letra) {
            // coisas do loop, como the_title() e etc
        }
    }
}

This code is far from great, but I believe it works as proof of concept. Basically, it fetches all posts from its base, and compares the first letter of each of them with a letter that you establish previously. For more complex or even more performative queries, you can play around with $ wpdb .

  

Show 20 articles

There is a proper parameter to this:

$args = array(
    'orderby' => 'title',
    'order' => 'ASC',
    'posts_per_page' => 20 //retorna o número de posts desejados
);

In query string format:

/?orderby=title&order=asc&posts_per_page=20