Enrollment (with two L’s) is standard in American English. Both words mean the act of signing up for services or the number of people signed up for a given service.
January 15: Open Enrollment ends — last day to enroll in or change Marketplace health plans for the year. After this date, you can enroll in or change plans only if you qualify for a Special Enrollment Period. February 1: Coverage starts for those who enroll in or change plans December 16 through January 15 and pay their first premium.