Total number of distinct numbers that can be represented using $n$ bits $=2^n.$
In case of unsigned numbers these corresponds to numbers from $0$ to $2^n -1.$
In case of signed numbers in $1's$ complement or sign magnitude representation, these corresponds to numbers from $-(2^{n-1}-1)$ to $2^{n-1}-1$ with $2$ separate representations for $0.$
In case of signed numbers in $2's$ complement representation, these corresponds to numbers from $-2^{n-1}$ to $2^{n-1}-1$ with a single representation for $0.$