I'm pretty sure it was 10 postcards in total, 7 unannounced and 3 already announced / confirmed. With it being 5-5 now with Invasion Of The Body Snatchers and Sisters having been found.
Edit: Not quite sure how to link a specific Facebook post, but here we go anyway:
Clicky.
I assume I'm reading it right and that Squirm and Deranged possibly aren't postcards, as Arrow tweeted "1 down 6 to go!" yesterday when someone posted a scan of the Invasion card. Unless Arrow aren't aware we know about Squirm and Deranged, which I guess is a possibility for Arrow in its current state. But you'd kind of assume we know since they posted a picture with them on, and loads of people have been mentioning them.