<?php

add_filter('widget_tag_cloud_args','set_number_tags');
function set_max_tags($args) {
    $my_args = array(
        'smallest' => 12,
        'largest'   => 12,
        'unit'  => 'px',
        'order' => 'ASC',
        'number' => '15');
    $args = wp_parse_args( $args, $my_args );

    return $args;
}