I see a report from 2012 which cites 500 Americans, out of a total 21,000 (https://www.aclu.org/national-security/factsheet-aclus-chall...), and a report from 2014 which cites 47,000 total (https://firstlook.org/theintercept/2014/08/05/watch-commande...), so I would assume the number of U.S. citizens on the no fly list is at least double the figure you list.
I see a report from 2012 which cites 500 Americans, out of a total 21,000 (https://www.aclu.org/national-security/factsheet-aclus-chall...), and a report from 2014 which cites 47,000 total (https://firstlook.org/theintercept/2014/08/05/watch-commande...), so I would assume the number of U.S. citizens on the no fly list is at least double the figure you list.