If you follow Alan's link to the FAA FAQs it clearly states:
If you operate your UAS exclusively under the Special Rule for Model Aircraft, you can voluntarily use the web-based registration process to register once and apply your registration number to as many UAS as you want.
Unmanned aircraft flown not as model aircraft must be registered individually by the owner, and each registration costs $5. Registrants must supply their name, address, and email address, in addition to the make, model, and serial number (if available) for each UAS they want to fly.
So, if you fly strictl