"8 is one of the only" is a little off because it doesn't have an object, it relies on that being implicit from the prior sentence. "Only" here is an adjective,

"8 is one of the only" is a little off because it doesn't have an object, it relies on that being implicit from the prior sentence. "Only" here is an adjective, not a noun. The usage is a little awkward. It would be more correct to say "there are only 4 positive integers less than 10. 8 is one of them."