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?