Correction: If a NFA has $n$ states, the DFA can have at most $2^N$ states.
This is because each state of the NFA can be a part of the DFA or not - thus we have two choices for each state and $n$ such choices to make, thus giving us $2^N$ states.
Every DFA is NFA.....
let us say NFA has N states
when we convert NFA to DFA
in worst case we get 2^N states in DFA