I want to make a deck of GUI cards appear on screen, stacked on top of each other.
But what if the amount of cards in one place is more than 10? How do I deal with this?
Unfortunately, ZIndices can only be 1-10. I think that the solution to your problem might be creating the cards in order of how you want them to be displayed. Each card you create will be placed on top of the last one created.
The max value for the ZIndex property of a GuiObject is 10, if you go over 10, the ZIndex property will automatically set the value to 10.