You have to override the below file in your theme or custom module.
vendor/magento/module-catalog/view/frontend/templates/product/view/addtocart.phtml
After overriding, you need to change the input to select as I do below.
<div class="field qty">
<label class="label" for="qty"><span><?= $block->escapeHtml(__('Qty')) ?></span></label>
<div class="control">
<select name="qty" id="qty" class="input-text qty" data-validate="<?= $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>">
<?php for ($i=1; $i < 50; $i++) :?>
<option value="<?= $i?>"><?= $i?></option>
<?php endfor;?>
</select>
</div>
</div>
After that clear cache and check. Let me know if you still facing any issues.
Thanks.