If you are talking about an atmega328p then the maximum supported clock speed @ 3.3V is 13.33MHz. In practice, you may get away with using a 16MHz crystal. The crystal itself doesn't care whether you run the microcontroller from 5V or 3.3V.
8MHz will be easier as you won't have to compile a new bootloader, there are pre-compiled ones for 16 and 8MHz, but not 12 AFAIK
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