The quantity will depend on how much meat you put in each taco. A moderate amount of 1/3 cup per taco will give you approximately 12 tacos for a 2 lb roast.
Prep the slow cooker with a liner for easy clean up (see note). Place the beef inside the lined slow cooker.
Add the remaining ingredients into the slow cooker.
Add water to the slow cooker until the beef is completely covered.
Place the lid on the slow cooker and cook on low for at least six hours.
When done cooking, remove the beef from the slow cooker. Trim off any excess fat and place the meat into the bowl of a stand mixer with a paddle attachment. Turn on medium speed to shred the beef, which will only take a few seconds.
Add some onions and juice from the slow cooker to the bowl and mix in with the beef using a spoon.
Notes
You need slow cooker liners in your life! Disposable liners that make clean up so incredibly easy! Find them here!Chuck roasts will vary in size. Most of the time I end up with one around 3 pounds, so just adjust the rest of the ingredients as needed to be proportional to the size of the roast.