Nobody knows exactly where or how the yogic postures originated.
5000 years old that show human figure, possibly priests or deities, seated in a familiar cross-legged posture. Just as nobody knows where the postures came from, so also nobody knows how many there are. A Brazilian Yoga teacher has published a book featuring over 2,000 postures. The postures and their names no doubt evolved alongside each other over several thousand years. The names have various kinds of relationships to the postures.