getChatMemberCount(chatId, form = {}) {
    form.chat_id = chatId;
    return this._request('getChatMemberCount', { form });
  }
Use this method to get the number of members in a chat.

@param {Number|String} chatId  Unique identifier for the target group or username of the target supergroup
@param {Object} [options] Additional Telegram query options
@return {Promise} Int on success
@see https://core.telegram.org/bots/api#getchatmembercount