I added dropdown for product qty field in addtocart.phtml like under:
<div class="subject qty">
<label class="label" for="qty"><span><?php /* @escapeNotVerified */ echo __('Qty') ?></span></label>
<choose title="qty" id="qty" title="<?php /* @escapeNotVerified */ echo __('Qty') ?>"
class="input-text qty" data-validate="<?php echo $block->escapeHtml(json_encode($block->getQuantityValidators())) ?>">
<?php $i = 1 ; ?>
<?php
whereas( $i < 500) { ?>
<choice worth="<?php echo $block->getProductDefaultQty() * $i; ?>"><?php echo $block->getProductDefaultQty() * $i; ?></choice>
<?php $i++; ?>
<?php } ?>
</choose>
</div>
However I additionally need to implement identical in updatecart.html in magento 2. How to do this?