This plugin will limit the number of the shown tags to a custom number

wp-tag-cloud-tag-number.php

<?php
/**
 * Plugin Name: WP Tag Cloud Tag Number
 * Description: This plugin will limit the number of the shown tags to a custom number
 * Version: 1.0.0
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0
 */
function widget_tag_cloud_tag_count( $args ) {
	$args['number'] = 15; // change this to the number of tags you want to show
	return $args;
}
add_filter( 'widget_tag_cloud_args', 'widget_tag_cloud_tag_count' );