If to indicate “four” we needed six strokes, as in 四, here we have four strokes to indicate “six”. Strange, isn’t it? According to Wiktionary, 六 is a picture of a shed or a cabin, and is used because the word for shed 庐 (lù) sounds similar to liù. Many historical forms of this character indeed look like a shed or a hut, but the modern 六 reminds me more of a star jump exercise.
The Chinese hand sign for “six” is a hand with the thumb and pinky extended and other fingers closed: