When slow-cooking frozen chicken in a crock pot, can I stack them up and cook many at once?

Not a good idea. You want the water up to temp quickly or you are creating a nice warm bath for bacteria to multiply like crazy. Even though the hot water would kill the bacteria eventually, there are some nuero-toxins created as a byproduct of the bacteria multiplying that are not destroyed by heat. I would strongly recommend that you give them about 20-25 minutes in a 350 F oven, then transfer them to the crock and add boiling water. It is more than your crock can safely handle from frozen or raw. But the headstart in the oven and boiling water will get you into the safe zone quickly. Using the oven first gives them more flavor too.