Edit: I just saw every crafting book has 0's, guess I'll go to the bug tracker.
Well, I could be wrong (and you devs out there, correct me if I am), but this 0 indicates that you can, for example, combine 65 cut barberry roots and 65 mugs of hot water into 65 barberry tea mixes (not sure if this particular example works, but hopefully you get the general idea).
There weren't 0's like this before they implemented combining/working on enough ingredients that you could create more than one item in one go, but they're there now, and are
not a bug.
Once again, this is just as far as I know, and I could be wrong.