Visually, every emoji is one character. Technically, many emojis are composed of multiple Unicode code points (a base emoji plus a skin-tone modifier, for example). On platforms that count code points, a 👨👩👧👦 family emoji can count as 7+ characters.
SMS gateways encode emoji as 2–4 bytes, which means a single emoji can split a 160-character SMS into a multipart message. Always verify on the destination platform.