To show multiple posts you need to change the $query_string global variable for the loop running.
Use the function query_posts() before the loop like this:
<!-- Changes the query_string for the front page. -->
<?php query_posts( 'posts_per_page=3&cat=13' ); ?>
<?php if ( have_posts() ) : ?>
<?php /* The loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
<?php get_template_part( 'content', get_post_format() ); ?>
<?php endwhile; ?>
<?php else : ?>
<?php get_template_part( 'content', 'none' ); ?>
<?php endif; ?>
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Read This – https://codex.www.ads-software.com/Function_Reference/query_posts
