We have no less than a thousand students in our College.
1. not less
2. no less
3. no fewer
4. No improvement
(c) fewer (Det., Adj.) : not many used for Countable Nouns.
Hence, no fewer is the right usage.
less (Det., Pro.) : a smaller amount of used for Uncountable Nouns
No less than (Idiom) : used for emphasizing a large