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Large cavern spaces are enough really, I get so many mimic spawns I don't even know.

Long flat places.

A technique that is good also is to go into the Underworld (if you have a long bridge from fighting the Wall of Flesh) and to run across it. Shadow Chest mimics will frequently pop up and increase the chances of getting a Cross Necklace.

Mimics usually popped out for me in humongous quantities when I was doing Chlorophyte farms, boss arenas for Plantera or when farming stuff, or less when just running in bigger caves. Humongous - at least a hundred or two per hour ( which has effectively gotten me at least a few dozens of platinum coins on every run ), so I'd say you should be more than fine.

Jungle has the highest spawn rates and a lot of spaces on top of that.