I found 3 un-numbered amiibo villager cards in my collection: Rosie, Goldie and Stitches.

At the back, they say Animal Crossing Amiibo Festival so they obviously came with my Amiibo Festival game.  On the front where the number would normally be, there's just the green leaf logo.

I tried to scan them into my game and they worked just like the normal numbered cards -- I can invite them too!

Does anyone know if these are part of a series, and how many other cards would have been in that series?  It would be great to track down and collect these un-numbered cards.

I do believe those were only available with the game, and there aren't any others to them. There are other special promo cards that were released in Japan, but those are the only other un-numbered cards I can think of.