Basically 1 per account. It'll disappear from the store for all characters once you buy one.
Unless they've changed something, you should still see a graphic and a description for it; it'll just be greyed out. Have the people reporting this checked their 'Legacy Unlocks' tab in the C-Store.
Formerly known as Armsman from June 2008 to June 20, 2012
PWE Drone says: "Your STO community as you have known it is ended...Display names are irrelevant...Any further sense of community is irrelevant...Resistance is futile...You will be assimilated..."