Bread - Liquid proportion in french toast
As Adele's comment suggests, it's really milk+eggs that matters (along with any other significant liquids in your recipe), not just milk.

The type of bread might have some effect: denser breads might not have as much room to take up liquid. But most people use some sort of reasonably light bread, so it can get a good amount of the custard in it - that's kind of the point. Nothing too open, like ciabatta, but also nothing too dense. So really, the amount of liquid is just the amount of free space in the bread, and that doesn't vary that much from bread to bread. If you're asking this in the context of looking at recipes, and the recipes in question specify a number of slices, the size of your loaf of bread and thickness of slices will far and away be the biggest factor.

What does have a bigger effect is how full the bread gets. A lot of people make it by dipping a slice of bread in the liquid mixture for a few seconds on each side, enough to soak up a decent amount of liquid, right before frying in a pan. This might not be long enough to really let it soak in, so it'll use less liquid. But I'll go out on a limb and say most people like it if the custard is thoroughly soaked into the bread. The answer you linked to is a great way to do that. You can also make it by soaking the bread for a longer period in the fridge (an hour to overnight). It'll have time to take up a bit more liquid that way. (You can then fry it as normal, or bake it to get something similar to bread pudding, except in slices.) Either way, the important thing is that the bread pretty much gets saturated.

So as long as you're really letting it soak in, there aren't really a lot of variables: the bread will get full of the custard mixture. If it's critical to plan ahead and get quantities exactly right, find a recipe you really trust. Otherwise, just wing it - if you're dipping the bread in, just mix up a batch, keep going until you run out of liquid, and if you want more, mix up a bit more. You can call this trial and error if you like, but honestly, it's what I do every time. It'll come out well if you let the bread soak up enough.