Can't we drop the 'ν' here? Eg: Κλείσε τη πόρτα
No, because when the next word starts with vowel or κ, π, τ, μπ, ντ, γκ, ξ, ψ the use of -ν is compulsory!
Is this used for plural or singular?
Κλείσε OR κλειστε ???
Either one is correct. "Κλείσε" is you singular and "Κλείστε" is you plural/and or formal.
Both are accepted along with "Κλείνε" you singular and "Κλείνετε" you plural/and or formal just another way of saying the same thing.