It’s just math. Just add everything up. You can certainly do it by hand and go by nutrition facts labels, but it’s easier to put it into a calculator that’ll do the math for you. Cronometer is a good tool for this.
To calculate what’s in a serving size, it’s just division. Let’s say you batch-cooked a dish that adds up to 1000 calories total. In four serving sizes, you’re looking at 1000/4=250 calories each.