First, let me be very clear. 'can' and 'should be' are two entirely different things.
To be sure that nobody reading your post gets the wrong impression, exodons should not be kept in a 10g tank
I do not suggest keeping exodons in a group of less than 10 for aggression reasons. With that in mind, you are looking at an average of 3"/each. with the 1"/g theory, you're looking at a 30g minimum for exodons. So for 12, it would be around the same. A 40B has a nice footprint and would provide more than enough room for a dozen exodons.
With more understanding of things like water chemistry, filtration, water parameters, nitrogen cycle, shoaling characteristics, GH hormones/ stunting, and all that other good stuff, you will learn how to responsibly bend the inch/gal "rule", but this is not a method commonly advocated.
In the end. More space = Better.
So when you ask me what the "optimal" tank for a dozen exos would be, its hard to say. 40 will be more than enough, 75 would be even better, hell, 1500gal would be sweet too. So whatever you are able to get and properly set up and maintain, would be the "optimal" tank.