Numbering author posts in posts query

You could have an authors totals array and on each iteration you would increment that author’s key:

<?php foreach ( $myposts as $post ) : setup_postdata( $post ); ?>
    <?php $author_count = isset( $authors_totals[ get_the_author_meta('ID') ] ) ? int_val( $authors_totals[ get_the_author_meta('ID') ] ) + 1 : 1; ?>
    Date: <?php the_date('d/m/Y'); ?> 
    Title: <?php the_title(); ?> 
    Author: <?php the_author(); ?>: <?php echo $author_count; ?>    
<?php endforeach; ?>