A basic batch of royal icing is 2 lbs powdered sugar and 5 large egg whites.
A basic batch based on that amount of sugar and egg whites will top coat approximately 4 dz - 3” cookie. You could probably get more cookies top coated out of that amount, but it’s best to estimate on the conservative side. It’s better to have more icing supplies than not enough.
Then factor additional icing for the design work. You know the cookie shape, size, and the design. Do you think out of a standard batch of royal icing you’ll need an additional 1/4 batch for the design? An additional 1/2 batch for the design? The more intricate the design, the more RI needed. I can’t estimate that for you.
So let’s say you’re doing the standard 2 pounds of powdered sugar, 5 egg whites per batch.
And you decide an additional 25% is enough for the design work.
2 lbs powdered sugar + 8 oz (extra 25%) = 2 1/2 lb powder sugar per batch
But of course it all depends on your recipe, your cookie size, shape, and the design.
If you use meringue powder then I have no clue. I’ve never used meringue powder, so I cannot give you any guidance.
If you make decorated cookies on a regular basis, you should routinely weigh an undecorated and an iced cookie in every batch. After a while you’ll begin to get a good sense of how much icing you need per cookie for the various sizes and designs. Then you’ll be able to narrow your estimates down to pretty close to your exact needs.
It’s amazing how quickly you’ll start to memorize your process and ingredient needs. I bake a lot from my head. I know I need 15g of flour per inch of pie plate for my pie crust. That goes up to 20 g of flour per inch for deep dish. If I’m going to make a batch of chocolate chip cookies I know I want to use 240g of flour per batch. if I’m going to bake a loaf of bread, I know I’ll need 400 g of flour per loaf for my favorite pan.
All of this will become second nature to you as time goes by.