I <3 Pizza
Thirty-two heartaroni were enough to cover this 10-inch-diameter pizza. But you might want to cut 40 just in case.
Stack and Slice
It's much more efficient to cut a stack at once rather than individually. (In fact, it's difficult to cut through just one slice.) Stack them no more than 8 high (it starts getting unwieldy after that.) Using steady pressure, push the cutter through the stack as evenly and level as possible. It goes through without much struggle.
Hammer It Home
The cookie cutter's not razor sharp, so the bottom slice in the stack will give you some trouble. Use a sturdy flat surface to pound the cutter through it all. (I use my 1-cup dry measure, but you could probably use a meat mallet, bwahahahah.)
Flip the stack over, with the cutter embedded in it, and hammer from that side, too. This should ensure that the metal has passed all the way through the meat.