Limit the Number of Multisite Registrations

I think I deleted my post somehow, so I will post it again.

Put the code in a plugin.

This code changes the message, hides the form and makes it to not work for users with at least 5 sites that are not super admin.

Let me know if you have questions. You will need to test it and improve it more, since I did it quickly and not perfectly.

if ( 'wp-signup.php' === $GLOBALS['pagenow'] || '/wp-signup.php' === $_SERVER['PHP_SELF'] ) {
    add_filter( 'gettext', 'my_change_blog_signup_function', 20, 3 );
    add_action( 'wp_head', 'my_custom_css_for_wp_signup' );
    add_filter( 'wpmu_validate_blog_signup', 'my_validation_for_wp_signup' );
}

function my_change_blog_signup_function( $translated_text, $untranslated_text, $domain ) {
    if ( $untranslated_text == 'Welcome back, %s. By filling out the form below, you can <strong>add another site to your account</strong>. There is no limit to the number of sites you can have, so create to your heart’s content, but write responsibly!' && ! current_user_can( 'manage_network' ) ) {
        $translated_text = 'Welcome back, %s. By filling out the form below, you can <strong>add another site to your account</strong>. You can have up to 5 sites.';
        $user_blogs = get_blogs_of_user( get_current_user_id() );
        if ( count( $user_blogs ) >= 5 ) {
            $translated_text .= '<br> <b>You cannot add more sites!</b>';
        }
    }
    return $translated_text;
}

function my_custom_css_for_wp_signup() {
    if ( is_user_logged_in() ) {
        $user_blogs = get_blogs_of_user( get_current_user_id() );
        if ( count( $user_blogs ) >= 5 && ! current_user_can( 'manage_network' ) ) {
             echo '<style> #setupform { display: none; } </style>';
        }
    }
}

function my_validation_for_wp_signup( $content ) {
    if ( is_user_logged_in() ) {
        $user_blogs = get_blogs_of_user( get_current_user_id() );
        if ( count( $user_blogs ) >= 5 && ! current_user_can( 'manage_network' ) ) {
            $content['errors']->add( 'my_awesome_error', 'limit to 5 sites' );
        }
    }
    return $content;
}