Hopefully this benefits someone, because it confused me until I read up: From what I understand, and don't take this as gospel, Zahl is used for stated numbers/figures. Like, "The NUMBER of provinces in Canada is 10."; whereas Anzahl is unstated numbers, like "Canada has a NUMBER of provinces."; whereas Nummer is an assigned number, like a telefonnummer (phone) or a hausnummer (address).
After googling it http://forum.wordreference.com/showthread.php?t=1206849 A more correct translation of Zahl seems to be "digit", цифра
That's what i gathered, but digit didn't work for me. Does it have a broader definition?
If you are referring to digit, it can also mean "finger", maybe that is why Duo didn't like it.