To operate a model aircraft (UAS) in the United States under the recreational rules spelled out in 14 CFR Part 107, the pilot must register as a recreational UAS pilot through the Federal Aviation Administration. This registration is for the pilot, not for each individual UAS, however, the registration number must be visible on each aircraft flown by the pilot.
Below is the link to the FAA Drone Zone where you can register or renew your UAS certificate.