Add has-\d-columns class to columns blocks after 5.3 update.

functions.php

<?php
/**
 * Add has-\d-columns class to columns blocks after 5.3 update.
 * 
 * @param string $block_content The block content about to be appended.
 * @param array  $block         The full block, including name and attributes.
 * 
 * @return string 
 */
add_filter( 'render_block', function( $block_content, $block ) {
	// Check if we have no columns block.
	if ( $block['blockName'] !== 'core/columns' ) {
		return $block_content;
	}

	// Check if the columns block already has a has-\d-columns class.
	if ( preg_match( '/<div class="(?:.* )?(wp-block-columns (?:.*)has-\d-columns|has-\d-columns (?:.*)wp-block-columns)(?: .*)?"(?:.*)>/', $block_content ) === 1 ) {
		return $block_content;
	}

	$columns_count = count( $block['innerBlocks'] );

	// Add the class to the markup.
	$block_content = preg_replace( '/<div class="(.* )?wp-block-columns( .*)?"(.*)>/', sprintf(
		'<div class="$1wp-block-columns has-%d-columns $2"$3>',
		$columns_count
	), $block_content );

	return $block_content;
}, 10, 2 );

This version is tested in WordPress 5.7 and contains some fixes (mainly to support also custom classes for the columns block):

add_filter( 'render_block', function( $block_content, $block ) {
	// Check if we have no columns block.
	if ( $block['blockName'] !== 'core/columns' ) {
		return $block_content;
	}
	// Check if the columns block already has a has-\d-columns class.
	if ( preg_match( '/<div class="(?:.* )?(wp-block-columns (?:.*)has-\d-columns|has-\d-columns (?:.*)wp-block-columns)(?:.*)?"(?:.*)>/', $block_content ) === 1 ) {
		return $block_content;
	}
	$columns_count = count( $block['innerBlocks'] );
	// Add the class to the markup.
	$block_content = preg_replace( '/<div class="(.* )?wp-block-columns([^"]*)"(.*)>/', sprintf(
		'<div class="$1wp-block-columns has-%d-columns $2"$3>',
		$columns_count
	), $block_content );
	
	return $block_content;
}, 10, 2 );