Making how much batter will result in over-mixing?
The amount is only restricted by the capacity of the tools you have at hand.  The point is, don't mix too much.  If you have a bowl, filled to the brim, you will have to mix a lot more to get all of the ingredients incorporated.  If that same bowl is half full, a few quick strokes is all it takes to mix. Over-mixing risks building the gluten structure too much, which results in a &quot;tougher&quot;, less light, final product. Bakeries, restaurants, and catering services probably do more at once than most people do at home...it is just that they have larger capacity tools.