It appears this is not an option currently available in n8nās Telegram node. But from checking at Telegram Bot API it seems to me you could make an HTTP POST request to https://api.telegram.org/bot123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11/getChatMemberCount where:
chat_id is a body parameter
bot123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11 is your Telegram API token