I find it difficult to count to ten in French: un, deux, trois, quatre, cinq, six, sept.
I can't say the next one because I have a "huit" allergy.
Hey, listen up, chucklehead! The joke is that "huit" sounds like "hate" in English. So the dude is saying he can't say "huit" because he "hates" allergies. And based on the allergies you got, bet you hate them too!