The "card drops remaining" -number is misleading. It doesn't actually show how many drops are remaining, but how many drops you have gotten so far.
This is most likely because they only count how many cards you've got and are using the same template as other badges, but didn't bother to change the wording on it or reverting the number in any way.
The actual number of cards you can get is 3 times the number of sale days so far, minus the number you've already got. I'm not sure if you can get the cards retroactively. (If you didn't get all 3 on day one, can you get them on day two?)