WooCommerce dropdown product quantity. It also correctly adds the correct minimum quantity when pressing the Add to Cart in the Shop page.

functions.php

/** Initial code from: https://gist.github.com/kloon/6495019 
As the code did not fully work I received help from Helga the Viking 
with this gist: https://gist.github.com/helgatheviking/ff8792fbb12f5c5367c816b8a46c70ad
 * Change the quantity input to select.
 * @param  array           $args Args for the input.
 * @param  WC_Product|null $product Product.
 * @param  boolean         $echo Whether to return or echo|string.
 * @return string
 */

/* My custom version. Minimum quantity of 10. Max of 500. */
function woocommerce_quantity_input( $args = array(), $product = null, $echo = true ) {
	if ( is_null( $product ) ) {
		$product = $GLOBALS['product'];
	}
	$defaults = array(
		'input_id'     => uniqid( 'quantity_' ),
		'input_name'   => 'quantity',
		'input_value'  => '10',
		'classes'      => apply_filters( 'woocommerce_quantity_input_classes', array( 'input-text', 'qty', 'text' ), $product ),
		'max_value'    => apply_filters( 'woocommerce_quantity_input_max', 500, $product ),
		'min_value'    => apply_filters( 'woocommerce_quantity_input_min', 10, $product ),
		'step'         => apply_filters( 'woocommerce_quantity_input_step', 10, $product ),
		'pattern'      => apply_filters( 'woocommerce_quantity_input_pattern', has_filter( 'woocommerce_stock_amount', 'intval' ) ? '[0-9]*' : '' ),
		'inputmode'    => apply_filters( 'woocommerce_quantity_input_inputmode', has_filter( 'woocommerce_stock_amount', 'intval' ) ? 'numeric' : '' ),
		'product_name' => $product ? $product->get_title() : '',
	);
	$args = apply_filters( 'woocommerce_quantity_input_args', wp_parse_args( $args, $defaults ), $product );
	// Apply sanity to min/max args - min cannot be lower than 0.
	$args['min_value'] = max( $args['min_value'], 10 );
	$args['max_value'] = 10 < $args['max_value'] ? $args['max_value'] : 500;
	// Max cannot be lower than min if defined.
	if ( '' !== $args['max_value'] && $args['max_value'] < $args['min_value'] ) {
		$args['max_value'] = $args['min_value'];
	}
	ob_start();
	if ( $args['max_value'] && $args['min_value'] === $args['max_value'] ) {
		
		echo '<div class="quantity hidden">
			<input type="hidden" id="'. esc_attr( $args['input_id'] ) .'" class="qty" name="'. esc_attr( $args['input_name'] ) .'" value="'. esc_attr( $min_value ) .'" />
		</div>';
	
	} else {
		/* translators: %s: Quantity. */
		$label = ! empty( $args['product_name'] ) ? sprintf( esc_html__( '%s quantity', 'woocommerce' ), wp_strip_all_tags( $args['product_name'] ) ) : esc_html__( 'Quantity', 'woocommerce' );
		
		echo '<div class="quantity">'; 
		do_action( 'woocommerce_before_quantity_input_field' );
			
		echo '<label class="screen-reader-text" for="'. esc_attr( $args['input_id'] ) .'">'. esc_attr( $label ) .'</label>';
		
		$options = '';
		for ( $count = $args['min_value']; $count <= $args['max_value']; $count = $count + $args['step'] ) {
			$options .= '<option value="' . $count . '" '. selected( $args['input_value'], $count, false ) . '>' . $count . '</option>';
		}
		echo '<div class="quantity_select"><select name="' . esc_attr( $args['input_name'] ) . '" title="' . _x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ) . '" class="qty">' . $options . '</select></div>';
		do_action( 'woocommerce_after_quantity_input_field' );
		
		echo '</div>';
		
	}
	if ( $echo ) {
		echo ob_get_clean(); // WPCS: XSS ok.
	} else {
		return ob_get_clean();
	}
}



/* Clicking "Add to Cart" in the Shop page added 1 quantity. As I have a minimum of 10 I had to add the following
code from Helga for it to work correctly. The following will add quantity of 10 to the cart. */
function kia_min_quantity( $args, $product ) {
    $args['quantity'] = 10;
    return $args;
}
add_filter( 'woocommerce_loop_add_to_cart_args', 'kia_min_quantity', 10, 2 );
// add_filter( 'woocommerce_quantity_input_min', 'kia_min_quantity', 10, 2 );								 

// Add a default value to all product page quantity dropdowns. Makes a check first to see that it is ! not in the cart.
function kia_default_quantity( $args, $product ) {
    if( ! is_cart() ) {
        $args['input_value'] = 30;
       }
    return $args;
}
add_filter( 'woocommerce_quantity_input_args', 'kia_default_quantity', 10, 2 );
								  
// OR use this code: Add a different starting value to multiple product page quantity drop downs. 
// Makes a check first to see that it is ! not in the cart.
function kia_default_quantity( $args, $product ) {
  if( ! is_cart() ) {
     if( 613 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #613 Product 1.
	$args['input_value'] = 70;
     }
     elseif ( 866 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #866 Product 2.
	$args['input_value'] = 60;
     }	
     elseif ( 145 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #145 Product 3.
	$args['input_value'] = 50;
     }		
     else $args['input_value'] = 30; // Condition: All other product ID's begin with the starting value of 30.
  }	  
	return $args;
}
add_filter( 'woocommerce_quantity_input_args', 'kia_default_quantity', 10, 2 );							  
								  

Updated code:

/* Change quanity input into drop down: https://www.businessbloomer.com/woocommerce-change-add-cart-quantity-drop/ */
function woocommerce_quantity_input( $args = array(), $product = null, $echo = true ) {

   if ( is_null( $product ) ) {
      $product = $GLOBALS['product'];
   }

   $defaults = array(
      'input_id' => uniqid( 'quantity_' ),
      'input_name' => 'quantity',
      'input_value' => '1',
      'classes' => apply_filters( 'woocommerce_quantity_input_classes', array( 'input-text', 'qty', 'text' ), $product ),
      'max_value' => apply_filters( 'woocommerce_quantity_input_max', -1, $product ),
      'min_value' => apply_filters( 'woocommerce_quantity_input_min', 0, $product ),
      'step' => apply_filters( 'woocommerce_quantity_input_step', 10, $product ),
      'pattern' => apply_filters( 'woocommerce_quantity_input_pattern', has_filter( 'woocommerce_stock_amount', 'intval' ) ? '[0-9]*' : '' ),
      'inputmode' => apply_filters( 'woocommerce_quantity_input_inputmode', has_filter( 'woocommerce_stock_amount', 'intval' ) ? 'numeric' : '' ),
      'product_name' => $product ? $product->get_title() : '',
   );

   $args = apply_filters( 'woocommerce_quantity_input_args', wp_parse_args( $args, $defaults ), $product );

   // Apply sanity to min/max args - min cannot be lower than 0.
   $args['min_value'] = max( $args['min_value'], 10 );
   // Note: change 20 to whatever you like
   $args['max_value'] = 0 < $args['max_value'] ? $args['max_value'] : 500;

   // Max cannot be lower than min if defined.
   if ( '' !== $args['max_value'] && $args['max_value'] < $args['min_value'] ) {
      $args['max_value'] = $args['min_value'];
   }

   $options = '';

   for ( $count = $args['min_value']; $count <= $args['max_value']; $count = $count + $args['step'] ) {

      // Cart item quantity defined?
      if ( '' !== $args['input_value'] && $args['input_value'] >= 1 && $count == $args['input_value'] ) {
        $selected = 'selected';
      } else $selected = '';

      $options .= '<option value="' . $count . '"' . $selected . '>' . $count . '</option>';

   }

   $string = '<div class="quantity"><span>Qty</span><select name="' . $args['input_name'] . '">' . $options . '</select></div>';

   if ( $echo ) {
      echo $string;
   } else {
      return $string;
   }

}

/* Add a different starting value to multiple product page quantity drop downs.
Makes a check first to see that it is ! not in the cart. */
add_filter( 'woocommerce_quantity_input_args', 'kia_default_quantity', 10, 2 );

function kia_default_quantity( $args, $product ) {
 if( !is_cart() ) {
 if( 20 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #613 Product 1.
 $args['input_value'] = 70;
 }
 elseif ( 19 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #866 Product 2.
 $args['input_value'] = 160;
 }
 elseif ( 11 == $product->get_id() ) { // Condition: Only apply these arguments to Product ID of #145 Product 3.
 $args['input_value'] = 50;
 }
 else $args['input_value'] = 20; // Condition: All other product ID's begin with the starting value of 30.
 }
 return $args;
}