When I am just testing samples for "try" vs. "nope nope nope," I will do 6-8 at a time and use the following spots on my arms: pulse point of wrist, crook of elbow, back of hand, halfway between crook of elbow and pulse point of wrist on my forearm. That's four spots for each arm, although I definitely smell like a mish-mash of things!

When I am testing for a longer period of time, I usually do two at a time--one on each wrist.