"The Yoga Sutras of Patanjali," written around 200 CE, give no instruction for specific yoga postures. Two manuals written in the Middle Ages, "The Hatha Yoga Pradipika" and "The Shiva Samhita," list 84 asanas but provide detailed instruction on less than two dozen. A late 17th century work, "The Gherand Samhita" by Chanda Kapali, claimed that there were hundreds of thousands of poses but provided detailed instruction for just 32 separate asanas. By the modern era 200 poses were outlined by the guru BKS Iyengar in his seminal text "Light on Yoga."
The style of yoga known as Ashtanga follows a set pattern of asanas and connecting moves called vinyasas. For instance, Sun Salutation A includes seven distinct postures and additional sequences of asanas for beginner, intermediate and advanced participants may include over 100 poses in a single, two hour practice session (See Reference 5). Bikram yoga, the original "hot" style of yoga, has 26 postures for beginners and encourages students to practice only these prescribed poses. Kundalini yoga, a style which incorporates chanting as well as movement, groups multiple asanas together into over two dozen clusters called "kriyas" or exercise sets, each designed to strengthen various parts of the body.
When it comes to counting the actual number of postures, variations will increase the length of that list. For example, marichyasnasa or "pose of a sage" has at least four different variations. Similarly, lotus pose can be half, full, bound, forward bent or embryonic -- with your arms threaded through your legs, hands holding your chin while balancing on your buttocks.
You can safely count five different families of asana: balancing, standing, seated, reclining on your back or laying on your stomach. The first family is characterized by balance challenges such as supporting your weight on one foot as in tree pose, across your hands and feet as in downward facing dog, or even on your hands only as in crow pose. The other families can be further sub-divided into how each asana arranges your backbone: forward bends, backward bends and twists.