I use this little aluminum case. Double sided, 8 microSD slots on one side, plus 1 SD slot where I keep spare cards. The other side holds 3 SD's and I keep my SD>microSD adapters on that side. I think it was 4.95 on ebay, free shipping from China, wait three weeks. It's a nice little case.
I drew a grid to number each slot in the case, and have a little notebook with info about each card (size, what's on it, last update, whether its a backup of another card, etc). On the back of the card I use a silver sharpie and number them using Roman numerals, so I can make sure I put them back in the correct slot. Roman numerals are all straight lines, so with a fine tip silver Sharpie you can write them pretty small and it's still legible. I should have turned one over when I took the photo so you could see the back.
As you can see cards #1 (OSMC) and #4 (Raspbian with 7" touchscreen) are currently in use.