I don't have millions of cards, but I have enough that keeping them organised is challenging. I also like to take sets of them out to play elsewhere, so bulky game boxes are annoying. So I designed a minimalist card holder framework and used that to create a small range of boxes. My aims were to use the minimum of material to hold the cards securely and avoid the need for any supports or brim so that they are easy to print.

1. A 60 card holder for a Magic deck. A rubber band holds them in place. Two of these fit neatly in the Magic The Gathering Rugged Card Box (https://www.thingiverse.com/thing:6013228), which I hadn't planned but is a nice bonus.

2. A 70mm card holder for organising my cards at home. That holds about 230 cards.

3. A 104mm card holder with a lid. That holds the entire set of Binding of Isaac: Four Souls - Second Edition cards. With a couple of rubber bands, that keeps the cards relatively safe when travelling.

4. A card divider for organising my cards in a larger box. It has a tab for writing/sticking a label on. The base means that cards will stay more or less where I have put them if I remove the adjacent stack of cards.

I ended up with the oval shape to provide stiffness and allow printing without supports.By total chance the shape ended up mirroring the back of the Magic cards almost exactly. I didn't even notice for ages. Happy coincidence that is now a nice feature.

You can scale the boxes up/down within reason on your slicer, to adjust the size to suit the size of stack/deck you want. Just don't go too far or you'll push the bridging too far and it won't print properly.

I have included the Blender file so that you can play around with it. Each box has three copies:

1. The basic curve-based shapes
2. A copy of those shapes, converted to meshes, with one that adds a Boolean which brings all the shapes together.
3. A complete mesh, ready to export to STL. If you want to do this step yourself. Duplicate the Boolean version of the faces from step 2, don't move it, then Apply each of the Boolean modifiers (press ctrl-A whilst hovering over them). That gives you the mesh for export to STL.