How much mozzarella? A ball is not descriptive enough

You all are so sweet to answer! Thank you for the responses!

Mozzarella pearls would work well also.

Agree with Nancy's comments. In this recipe, use as much or as little as YOU would like, customizing to your taste.

Given the way the recipe was written, size or measure doesn’t need to be exact.
If sold loose in brine, mozzarella balls is usually the size of a medium orange, about a cup.
If sold packaged, usually about 12 oz.
So estimate somewhere between those weights, use what you can find and/or what looks good as you scatter torn bits on the pizza.