You can adjust the quantity depending on how much you want to cook and how large your crockpot is. As a rule of thumb, use 1 cup of sauce for every pound of meat.
I get juicier chicken from my slow cooker when I cook chicken on low, not high. I recommend using low if you have the time.
Let the meat rest for 5 minutes before shredding. It will reabsorb some juices and be more tender.