How much black garlic to substitute for regular garlic in a slow-cooker?
Black garlic is not a substitute for regular garlic, you add black garlic to get a different result. I would never say use 2 cloves of black garlic for every 1 regular or any other ratio because the changes are so fundamental that they really are totally different ingredients. 

The question is how much black garlic flavor you want, and only you can answer that. I'd suggest that you taste it when it's close to being done and if you want more add more then.