columns-block-filter.php

<?php

/**
 * Render columns block with extra class
 *
 * @param string $block_content The block content about to be rendered.
 * @param array  $block        The full block, including name and attributes.
 * @return string
 */
function render_columns_block_class( $block_content, $block ) {

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

	$block_content = str_replace( 'wp-block-columns', 'wp-block-columns wp-block-columns-has-' . $columns, $block_content );

	return $block_content;
}
add_filter( 'render_block_core/columns', 'render_columns_block_class', 10, 2 );

This offered in response to this tweet by carrie dils: https://twitter.com/briancoords/status/1729589304593162558

For Christmas, I wish there was an auto-class applied to the Columns block indicating the number of child elements. Sure would be nicer than relying on content editors to add a custom CSS class.