How many combination of “n” bits are there in terms of n?
Each bit can be either 0 or 1, so you have two choices per bit. That gives you 2^n combinations. E.g. n=1 implies 2^1=2 states, n=2 implies 2^2=4 states. You could arrive at this by 1) making a lexicographic list of all the combinations or 2) using a formula from combinatorics.

Your second question seems to address representing integers with binary numbers. You must first settle on an encoding scheme before that question can be answered. A popular encoding scheme is two's complement. The maximum positive integer represented in 4-bit two's complement is 0111, which is 7.