To do something like this you need to loop through the layouts first and gather the information and then loop over the gathered information to show the list
<?php
$anchors = array();
if (have_rows('flex')) {
while (have_rows('flex')) {
the_row();
if (get_row_layout() == 'anchor') {
$anchors[] = get_sub_field('anchor');
} // end if anchor
} // end whild have rows
} // end if have rows
if (count($anchors)) {
?>
<div class="container">
<div class="row">
<div class="col-md-12">
<?php
foreach ($anchors as $anchor) {
?>
<a href="#<?php
echo sanitize_title($anchor); ?>" class="btn btn--fancy"><?php
echo $anchor; ?></a>
<?php
} // end foreach anchor
?>
</div>
</div>
</div>
<?php
} // end if anchors
?>
