More buttons for different number of copies
When handling cards in a deck, you either have to click into the little text box and write number of copies, or manipulate the slider or you have to click the + / - buttons - but when doing multiple copies you would need to click multiple times (usually 4 times for playsets).
My idea would be to add one extra button on each side of the slider - one to remove all copies at once and one to set the number of copies to four.
Heres a picture of how it might look like:
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Good idea!