Once you have hit n numbers, the probability of getting a new number on the next spin is (38-n)/38. If the probability of an event is p, then the expected number of trials before it happens is 1/p. Thus the expected number of spins to get a new number, given that you already have n, is 38/(38-n). For example once you have hit 20 numbers the expected number of spins to get the 21st is 38/18=2.11. So the answer is the sum of the expected number of spins at each step: (38/38)+(38/37)+(38/36)+...+(38/1)=160.66.