With some recipes, you can just double up, and some recipes have tiered guides that will tell you the exact ingredients to use for the size cake you are looking for. However, if you don't have an exact recipe, then you'll be down to trial and error.
For a fruit cake, I would advise you find a tried and tested 12-inch recipe just for the number of ingredients that you are going to need and the price it will cost you to make it. (Unless you are happy to experiment with one of your favourite recipes of course).
No matter what you do though always do a trial run before the big day that way you can iron out any kinks before the actual event.
So now to the help:-
The shape of the cake - It makes a difference depending on what the shape of the cake is as to how much ingredients you will need. Square cakes need approx 20-25% more ingredients than round ones do.
Size of cake - For such a large cake you will need to bake it on a lower temperature anywhere from 130 - 150°C depending on if you have a fan assisted or conventional oven or, bake it on a very low gas mark.
Length of bake - I would definitely advise using a water tray to add moisture to your cake and line it.
The depth of your cake - Depending on the depth of your cake whether it be 2 inch/3 inch/4 inches deep will also affect the bake time. The deeper the cake, the longer the bake time.
Portion Size/ Cake Size - If you need to make a 12-inch cake for the number of guests would it not be worth doing a tiered cake using smaller tiers which will effectively take you less time to do?
Time: An 8-inch x 3-inch fruit cake takes me three and a half hours to bake, so you are looking at anywhere between 5-8 hours for the complete baking time on a large deep 12-inch cake tier. (Beware though because even though sometimes they look done they can be undercooked in the middle).
I don't want to overload you with tons of info but hope this gives you an idea of all the things you'll need to think about.
If you need anything else then please let me know.