The half and half amount is ranged in the recipe because some people like heartier, stiffer mashed potatoes while some like it to be a thinner puree. I like right in between, so I think I added 1 3/4 cups…but start with 1 1/2 and add from there!
The chicken base that I buy is called “Better than Bouillon”, and it comes in a small-ish jar in the same section as the boxed chicken broth in your grocery store. If you can’t find it, you can either cook the potatoes in boxed/canned chicken broth OR just skip this all together and cook them in salted water!
Find it online: https://www.thegarlicdiaries.com/brown-butter-mashed-potatoes/