how to add number for post ( not post id )

Hi Evryone
i need to add number to my post in index.php

example:

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my code in index.php

<article id="article" class="posts">
<?php if(have_posts()) : ?> <?php while(have_posts()) : the_post(); ?>
<h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2>
<?php the_excerpt(); ?>
<?php endwhile; ?>
<?php endif; ?>
</article>

please help me