Translation:He gets twelve books and thirteen plates.
I have the same problem and posted a question in the Duolingo forum. The rest of the sentence can be fine but any number word is marked wrong. Apparently it's a glitch in the voice recognition software. It affects other languages as well, but seems particularly bad for Dutch.
"ij" definitely does not sound like english "ay". English "ay" corresponds with dutch "ee" (veel mee beetje).
There really isnt a matching sound in english but closest is the vowel sound by/I/high. (Though in english it sounds like an a sound ending with a j "aj")
Im shocked to hear you saw this in several places (hopefully not sites teaching languages)