Actually, would look like this (untested):
'users' => $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->users INNER JOIN $wpdb->usermeta ON ($wpdb->users.ID = $wpdb->usermeta.user_id) WHERE 1 = 1 AND ( $wpdb->usermeta.meta_key = ' {$wpdb->prefix}{$blog_id}_capabilities' ) " ),
No need to escape $wpdb when using double quotes. :)
See the docs on $wpdb:
http://codex.wordpress.org/Class_Reference/wpdb#Class_Variables
http://codex.wordpress.org/Class_Reference/wpdb#Tables