I'm currently routing a board and need to supply 12A between a current sensing IC and an ESC. Due to size constraints I cannot have both the decoupling capacitor and IC on the same side of the board. Is it better to use a single large via to pass this trace between the top and bottom sides of the board, or would several smaller vias be better?
A single via that would happily carry 12A is a pretty big via for someone who says they don't have a lot of room. Have you checked out Saturn PCB toolkit?
Its via tool will give you an idea of the current handling vs temperature rise of a given via. You might want to consider filled vias too and think about your copper plating thickness etc.
As current capacity depends basically in copper width and plating thickness it will basically be the same to have, for example, two vias of 0,3mm diameter or one unique via of 0,6mm diameter, in both cases plating thickness being the same.
Here you can check an interesting article about this topic. At the end of the text there´s small explanation on this ("Multiple Vias").