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..."