Get subscribers count via the MailChimp API.

subscribers-count.php

<?php
/**
 * Get subscribers count via the MailChimp API.
 */
function mailchimp_get_subscribers_count() {
    $cache_key = 'mailchimp-subscribers';
    $api_key   = '';
    $username  = '';
    $dc        = '';
    $list_id   = '';

    $count = get_transient( $cache_key );
    if ( $count === false ) {
        $count = 0;
        $url = "https://{$dc}.api.mailchimp.com/3.0/lists/{$list_id}";
        $request = wp_remote_get( $url, array(
            'headers' => array(
                'Authorization' => 'Basic ' . base64_encode( $username . ':' . $api_key ),
            ),
        ) );

        if ( wp_remote_retrieve_response_code( $request ) == 200 ) {
            $body = wp_remote_retrieve_body( $request );
            $body = json_decode( $body, true );
            $count = absint( $body['stats']['member_count'] );
        }

        set_transient( $cache_key, $count, 12 * HOUR_IN_SECONDS );
    }

    return $count;
}