Depends on your songs, possibly. If you’d like to be able to have all 8 scenes on one view for instant switching, or a lot of effects available, or a lot of presets available in one view as you record, the FC12 would cover that. It would also be handy if you wanted, say, several scenes, a couple of effects, and a few presets all at once.
If you need less, do a lot of dubbing and punching in parts, or are short on space, the FC6 might be your partner. It’s literally half a FC12.
If you don’t know, I’d go 12. More unused switches beats adding outboard switches to make up for not enough, in my opinion anyway. On the other hand, you could pick up a used FC6, try it out, and get most or all of your money back if you need to sell and upscale.