I had to add “, ‘,’” to the filterBy to make it count as I want (found here):
<?php foreach($tags as $tag): ?>
<?php $counttag = page('blog')->children()->listed()->filterBy('tags', $tag, ',')->count(); ?>
<a href="<?= url($page->url(), ['params' => ['tag' => $tag]]) ?>">
<button type="button">
#<?= html($tag) ?> (<?= $counttag ?>)
</button>
</a>
<?php endforeach ?>
Thanks for support!