How to show more than 5 posts of the category?

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I hope you are talking about Learn More theme by Humble themes. Just downloaded this theme and checked source code.
File get_posts.php has an argument, which returns only 5 posts.
This file is located in theme folder: learnmore/inc/functions/get_posts.php

    $args = array(
        'orderby'    => get_theme_mod( 'kb_article_orderby', 'date' ),
        'order'      => get_theme_mod( 'kb_article_order', 'ASC' ),
        //here it goes
        'numberposts' => get_theme_mod( 'kb_article_count', 5 ),
        'category'    => $category->term_id,
    );

But I think you don’t need to change it in a source code. This theme has an options or theme page, where I see this text:

All Setting, Theme Options, Widgets and Menus are available via
Customize screen. Have a quick look or start customization!

So I recommend you to check customizer settings, you should find there an option to change the number of posts per category.

Update

how can I remove the “View all articles” sentence -under the posts-?

You can find this link in a layout-1.php file, which located: learnmore/template-parts/layout-1.php, lines 56-58. Remove this code or modify it, it’s up to you.

<a class="learnmore-category-link" href="<?php echo esc_url( get_category_link( $category ) ); ?>">
    <?php esc_html_e( 'View all articles →', 'learnmore' ); ?>
</a>