Normally I wouldn't post links to the Destiny forum on Bungie.net, since it's the filthiest hive of scum and villainy on the known Internet, but this thread caught my eye:
Potentially there are 8 more Destiny Cards to be released, if some things that have been seen in some of the page source code on the website are to be believed.
As we know 17 codes have hit the net so far (if you haven't redeemed the initial 17 yet, they're all in the link's OP and are multiple-use so go redeem them all for free content on game launch) and people are suspecting that, since no new cards were released at E3, that Bungie is holding off on releasing the remaining cards until Bungie Day (July 7th.)