Check Min/Max quantity for line items and stop save if it is over.

functions.php

<?php

/**
 * Hooked into `jgtb_allow_edit_qty_for_subscription`, this will check
 * the new quantity of items in the subscription and allow/disallow the 
 * edit.
 *
 * @param  string          $allowed      'yes' or 'no'.
 * @param  \WC_Subscription $subscription Instance of current subscription.
 * @return string                        'yes' or 'no'.
 */
function sp_custom_allow_edit_quantity( $allowed, $subscription ) {
	if ( 'no' === $allowed ) {
		return $allowed;
	}

	foreach ( $subscription_items as $item_id => $item ) {
		$old_qty = $item->get_quantity();
		// Get new quantity of item from request
		$new_qty = ( isset( $_REQUEST[ 'new_quantity_' . $item_id ] ) ) ? $_REQUEST[ 'new_quantity_' . $item_id ] : null;
		if ( null !== $new_qty && $new_qty != $old_qty ) {
			// TODO: Check product's Min/Max quantity and if the new quantity is over then set $allowed to false.
      $max_qty = '';
      if ( $new_qty > $max_qty ) {
        $allowed = false;
		}
	}

	return $allowed
}
add_filter( 'jgtb_allow_edit_qty_for_subscription', 'sp_custom_allow_edit_quantity', 10, 2 );