Golftec uses two sensors - upper back and your tailbone, have not used this system, information gotten through a conversation with them when I was looking to buy my son a gift certificate this past holiday season. The other system I have used, K-Vest (now known as K-Motion) used three sensors - upper back, tailbone and wrist. K-Motion has added a 4th sensor on the upper arm. Over 15 years ago I had my swing analyzed at the University of Pittsburgh Biomechanics lab - they used many sensors; high end systems like this are only at universities or maybe Hollywood. Video game developers used these sophisticated systems to capture golf swings for their Golf games (see the Tiger Woods golf game).
Another commercially available system is "mySwing Professional" which uses 17 sensors - lower body, upper body and arms.
If you want to practice, get a really cheap club at a second hand store and practice on that. Less stress that way. Remove the head, clean it, re-epoxy, etc. I posted the general process here.