How much milk should you steam at a time? Ideally the answer to this question would be, exactly as much milk as you want to use in your drink, but, of course, life never works out quite perfectly unless there is a lot of luck or a lot of skill guiding it along.
One should have the goal of steaming less and less milk as your skill develops until you are no longer wasting any milk, but this does take lots of practice and may not even be a feasible goal on some machines.
One of the reasons why it might be tough, other than that you might not guess the right amount of milk from the start is that steaming milk is much harder for very small quantities than it is for large quantities if you want to produce microfoam. For a macchiatto, where you might not even want an ounce of milk, you would need to be incredibly skilled to just foam the exact amount of milk you want, but even for a cappuccino, it is hard to steam the exact right amount of milk for a single one on a very powerful machine (this is one of the few advantages the category of smaller home semi-commercial machines have in steaming over the big commercial machines with bigger boilers).
Another issue is that you generally want a given volume of milk – just enough to fill the cup you are using, but as you steam milk, if you do it right it should expand quite a bit.
Let’s instead focus on the right amount of milk for a given pitcher. Ideally a pitcher usually works best when the milk comes up just below the bottom of the spout on the inside. You can certainly steam milk in a pitcher that is to big and you can even do it in a disposable paper cup, but it helps to use the right size pitcher if you are just starting out.
For a cappuccino or two, a 12 oz pitcher is a good size, for a latte this will work, too, but for more drinks you may need a bigger pitcher.
I use my 12oz pitcher almost all the time unless I am making hot chocolate for my kids. If I had a smaller pitcher I think I’d use it for smaller drinks, but I can get away with the 12 oz one so it is a good compromise.