Can't say I've been counting. I send them out, I post a note a week or so later saying to contact me if you don't receive it. I receive a random number in my letter box... Seems to work OK. No doubt some heading my way go missing in the post, others are never sent. As long as the numbers never sent are small, I don't give a whatsit: I'm not gonna sit down and check each postcard I receive off against the mailing list - life is far too short.
If someone is deliberately receiving and not sending, best of luck to them - I hope they find a few postcards suitable recompense for such a sad and pointless existence that it dictates that they indulge in such absurd behaviour.
They won't of course.