What can I do with 12 spare adhesive floor tiles?

Keep them, you might need them for future repairs.

Make sure you save some for repairs. Use as a shelf liner under your kitchen sink.

I Kathy, In years past, I've used these in the bottom portion of a kitchen and bathroom kitchen beneath the sink area to have an easy to clean area there. That may or may not be an idea for you to use. Also, you might possibly use them on shelves of an old bookcase or shelving in the garage.

I used extra adhesive floor tiles to make a matching backsplash in the kitchen and bathroom, to cover up a badly scarred tabletop, and to cover a hole in a door (don't ask!).....

Thank you all for your answers and responses. Im gratful.

I have used them under kitchen or bathroom sinks. Makes a nice protection for wood as we store lots of soaps, etc. under them ..And it makes it an easy clean up