Aha, there we are again with the difference between cardinal numbers (tal in Danish) and numerals (antal). :)
Numbers themselves are singular entities by definition, so "Five is less than eight" is the correct option. But if you start counting something, you get into the realm of plural objects: "Five apples are fewer than eight (apples)."
You have the same difference in Danish:
- Fem er mindre end otte.
- Fem æbler er færre end otte (æbler).