Why each function may be computed with circuit with 2^n gates?
The meaning of the statement is that each function can be computed with at most $2^n$ gates; it doesn't anything about the minimal number of gates needed to compute any function.

That said, you can always add dummy gates to increase the number of gates in your circuit without changing the function it computes. For example, you can replace one of the inputs $x_i$ to $x_i \lor x_i$ to increase the number of gates by 1.